Celeste

The B side cassette can be found from the same starting point as the blue crystal heart. Section 3, hollows.

Use the moving block to break down and then left for a new area where you find the cassete.

Not my video:

As for the actual chapter solution,
try the below guide (also not mine)

There will be a video below if you prefer video over text!!!

The B-Sides are unlocked after finding the cassette in the A-Side, or normal version, of their given level. So, for the B-Side of Chapter 6, starting from Section Three, climb up the two walls on the left to find a moving block guy. Slap his right side to get him moving, then slap his bottom side to get him to break the ground underneath and fall below. Follow him and slap his left side to have him break the wall on the left, revealing a new zone which holds the cassette tape. After collecting the cassette and finishing the level, you'll get your rundown of strawberries collected and total deaths, plus the unlock for the B-Side.

B-Sides are more difficult version of the given chapter, with the same theme and mechanics of the level, just turned up to 11. The sections are roughly the same length as their A-Side counterparts as well. Every B-Side ends with an alternating pink and blue platforming section with their Crystal Heart at the very end. Plus, they have a bangin' alternate version of the level's theme song. That should be everything, so let's get into it. I'll break each section down into their own parts so they're easily identifiable.

Section One

• You'll start on top of a moving guy, who you'll need to slap on the left side to get him moving. Slap his top as you come to the center of the room to fly up, then stand on his left edge. Collide with the ceiling and fall down the gap to slap his left side, then get back on top. Slam into the left wall and slap his top and adjust to his right side after you pass the brambles. Slam into the ceiling and fall down the gap to slap his right side, then quickly get on top and duck down to avoid the brambles overhead. Ride to the right wall and jump off to climb through the ceiling.

• Dash to the feather and fly up and curve around the wall to another feather. Fly down this time and curve under the next wall for another feather. Fly up with the last feather and weave between the brambles, then dash right out of the feather to get the green gem, then dash up-right to avoid the brambles below and get another green gem. Dash right into the last green gem and dash up-right to climb through the ceiling.

• Use the binoculars if you like. Jump to the left side of the bouncer and dash left to bounce off the top of the next bouncer to get as much height as possible. Dash up through the brambles and bounce off the right side of the bouncer, then dash up into the feather. Fly down through the gap in the brambles, then curve right to fly up through the small gap on the right and dash to the wall to climb through the ceiling.

• Jump onto the moving guy and slap his top. Ride him up and jump off the right side as he slams into the ceiling to clear the brambles ahead and fall to the moving guy below. Slap his right side and grab on to ride him right, then jump up into the feather to fly to the blue gem and curve down and around the brambles, ending the flight pointing up-right so that you fly over the top of the moving guy, then slap his right side and ride him back to the wall. Climb up and exit right.

• Jump down and dash left into the bouncer, sending you flying, then dash up and hit the right side of the overhead bouncer to be launched again. Dash up-right as you near the brambles, then hit the bouncer on his upper-right, then dash up to grab onto the wall overhead. Climb up and jump to bounce off the upper-left part of the bouncer below, then dash up-right over the single bramble and fall down to bounce off the upper-right part of the bouncer. Fly up and dash up-right to reach the end and exit right.

• Wait for the bouncer to move right and jump and dash right to bounce off its upper-right part. Fly across and dash to the crumbling platforms. Jump to the next one and jump right to bounce off the top of the bouncer. Fall back down and dash right after you clear the brambles, then bounce off the top of the bouncer again and wait to dash and land on the crumbling platform ahead until after you clear the brambles. Jump to the next bouncer and bounce off its top part until it starts moving up, then bounce off its right side to fly past the brambles and dash up-right to land on the crumbling platform. Jump and fall past the brambles, then dash right to the last bouncer and bounce off its top until you can comfortably dash up through the floor overhead to exit right.

• Jump down and slap the right side of the moving guy, then ride him to the wall and jump off to wall bounce to the feather, quickly circling the bramble to get the blue gem and fly back down to slap the moving guy's right side again. Hold on until the next wall, then jump and wall bounce to get the green gem and jump and dash into the right side of the bouncer to be flung to the wall ahead. Fall down and slap the right side of the moving guy again. Hold on until the next wall, then jump and wall bounce to the second moving guy and slap his right side. Ride him to the wall ahead and jump and wall bounce to get a green gem and grab onto his left side. Ride him slowly back to the opening overhead and jump and wall bounce to get the blue gem and clear the moving guy, then fall down and slap the first moving guy on his right side, collecting the blue gem at the same time. Ride him to the end of the zone and exit right into the next section.

Section Two

• Just walk and jump right to exit through the ground.

• Jump down through the gap in the brambles and dash right to fall into the first green gem. Dash left to fall into the second green gem, then dash right to fall into the third green gem. Dash right to fall into the fourth green gem, then dash left and fall to the left to weave through the brambles and reach the bottom to exit right.

• Jump down past the brambles and dash right to fall into the first green gem, then dash right again to fall into the second green gem and slap the moving guy on his left side. Slide down and let him slam into the wall, then drop to the wall and jump off to get the third green gem. Slap the next moving guy on his right side and jump off as he slams into the small block to be flung to the opposite wall. Slide down and jump left to clear the brambles below and reach the bottom. Exit right.

• Jump and dash to fall through the brambles, then bounce off the upper-left portion of the bouncer to be flung left and weave through two gaps in the brambles and dash right. Bounce off the left side of the bouncer to be flung across the screen, then dash to clear the last bit of brambles and fall to the bottom. Swim under the brambles in the water and exit right.

• Jump and weave right through three gaps in the brambles, then left through one gap and dash left to fall into the first green gem. Dash right to fall through a gap, then weave left to get the second green gem. Weave right through the next gap, then dash left to fall to the bottom and exit right.

• Jump and dash right to fall onto the head of a moving guy. Slap him and jump as he hits the block overhead, then dash left to fall into the first green gem. Dash right to fall into the second green gem, then dash into the left side of the moving guy and hold on tight. After he stops, jump to the left and slide down the wall. Jump and hold right to reach the bottom and exit right.

• Jump right and weave through the brambles to reach the first wall. Slide down and jump left to clear the next set of brambles down to the second wall. Jump over the brambles and grab onto the third wall, then jump right and dash up-right to clear the last set of brambles and fall to the bottom. Exit right.

• Jump and dash to fall into the first green gem, then dash left and weave right to fall into the second green gem. Dash right into the feather and fly left and curve down and right, aiming for the small wall to grab onto. Jump and dash left to fall into the third green gem, then dash right to fall through the gap and weave around the single bramble below to reach the bottom and exit right.

• Jump into the gap and dash right to get the first green gem, then dash up-right to clear the brambles and get the second green gem. Fall down and dash left to get the third green gem, then dash up-left to reach the wall and climb up. Jump to the fourth green gem and immediately dash up-left to get enough height and distance to clear the brambles, then weave through the brambles to get the fifth green gem below. Dash right into the sixth green gem, then fall into the small area below and dash up-right to get the seventh green gem. Dash right and fall down a narrow gap, getting the eighth green gem along the way, then dash left to fall into the ninth green gem and dash left again to fall through the narrow gap and reach the bottom. Exit right.

• Jump and dash into the first green gem, then dash up-right into the second green gem. Dash right into the third gem, then dash up into the fourth gem. Dash up-right into the fifth gem, then dash right into the sixth gem. Dash up-right into the feather and fly right to the end of the zone. Exit right into the next section.

Section Three

• Jump and dash into Badeline for the first time, then bounce off the falling wall on the left and dash right through the brambles to land on a jump pad. Bounce up and dash into Badeline for the second time, then bounce off the falling wall and dash right to land on another jump pad. Bounce up and dash into Badeline for the third time, then bounce off the small wall and land on top of the falling block. Jump and dash right to fall onto the jump pad and bounce up into Badeline, then dash right and exit right.

• Jump right into Badeline for the first time, then dash right and grab onto the falling wall. Quickly climb up and jump over it to get the green gem, then dash into Badeline for the second time. Hold right and dash up-right to clear the falling wall and the brambles ahead, then fall into the feather and fly between the brambles to exit right.

• Jump and dash to the first green gem, then dash up to the second green gem and right to the third green gem, then dash into Badeline for the first time. Dash up to land on top of the right falling block, then jump and dash up-right into the fourth green gem, dash right into the fifth green gem, then up-right into the sixth green gem. Dash into Badeline and dash left after you clear the brambles to get the feather. Fly around the falling block overhead and turn right once you clear the brambles up top to fly across the zone and hit Badeline for the last time in this zone. Land on the wooden platform, then exit right.

• Grab the feather and fly to the bottom blue gem, then circle to get the top blue gem and dash into Badeline for the first time. Grab onto the wall and wait for Badeline to shoot an orb, then jump and dash up-right to clear the brambles and fall into the feather between the brambles. Fly down to get the bottom blue gem, then circle around the center to get the top blue gem and aim to hit Badeline for the second time, sending you back into the feather again. Fly through the middle this time and follow the upper path to get the next two blue gems and aim for Badeline for the third time. She'll send you through the last blue gem, then fall down and dash through the green gems to get the last feather. Fly up and curve around the brambles to reach the end, hit Badeline for the last time for this zone, then exit right.

• Jump into the small gap in the brambles and dash left to fall down to the green gem. Jump and dash right so that you can bounce off the top of the bouncer and grab onto the wall overhead. Jump off and dash into Badeline for the first time, then bounce off the wall and fall down the hole to hit Badeline for the second time. Dash up-left into the green gem, then dash up-left again to get the feather. Fly around the brambles as you make your way down, then dash right to land on the crumbling platform ahead. Jump and dash right to hit the top part of the bouncer, then dash into Badeline for the last time for this section. Dash right and exit into the next section.

Section Four

• Jump and dash up-right to slap the left side of the moving guy, then bounce off him and off the wall to get the green gem above and grab onto his right side. Slap his right side and jump off as he slams into the small wall ahead, sending you flying to a green gem in a small gap of brambles. Dash left into the right side of the second moving guy, then jump off him as he slams into the small block to be flung ahead, getting a green gem along the way. Dash up-right into the next green gem, then dash up-right to clear the brambles and grab onto the right side of the third moving guy. Slap his right and jump off as he slams into the small block ahead, getting a green gem along the way. Dash up-right to slap the last moving guy on his left side, then quickly climb up to avoid the brambles ahead and jump to the last two green gems, where you'll dash up into the ceiling.

• Jump and dash to the first crumbling platform, then dash up to the next one and jump and dash up-right to clear the brambles and bounce off the wall to hit Badeline for the first time. Grab onto the wall and let it stop moving before you jump to the block ahead. Jump and dash right to hit the top of the bouncer, then dash into Badeline for the second time. Bounce off the top of the bouncer until the wall ahead comes back, then dash to its right side and let it throw you into Badeline for the last time for this zone. Dash up-right to avoid being squished by the last block and exit right.

• Jump down and dash left into the right side of the first bouncer, then dash up-left into the left side of the second bouncer to be flung towards Badeline for the first time. Bounce off the wall and dash into the feather, then fly around the brambles below and up the vertical shaft, curving around the end to fall to the blue gem below on top of the crumbling platform. Jump to the top of the bouncer and fly up to the blue gem in the small gap overhead, then fall back down onto the bouncer and fly up to get the other blue gem on the right of the last one. Fall down to the last blue gem, then get on the bouncer for the last time and dash through the new hole and hit Badeline for the last time for this zone. Exit right.

• Jump to the right and dash up-right to land on the first crumbling platform. Wait for it to fall apart to hold right to land on the platform below, then jump and dash into Badeline for the first time. Slide down the wall and jump and dash to the crumbling platform ahead, then jump to the next one and dash to Badeline for the second time. Bounce off the wall and fall to the moving guy below, giving him a slap on his right side. Jump off as he slams into the wall to be flung into Badeline for the third time. Fall down the wall and wait for the bouncer in the center to appear, then jump and dash for it, bouncing off its top. Dash for Badeline for the fourth time, then slide down the wall and wait for her to shoot. Jump across the crumbling platforms to hit Badeline for the last time for this zone. Dash right to exit right.

• Jump and dash into the feather to fly right, weaving between the upper and lower brambles. Fall off the end of hte feather and dash right to fall into another feather. Fly right and dash up out of the feather to slap the moving guy on the right side of his bottom, falling into the nearby feather to fly through the gap and curve right to avoid the brambles. Fall down past the bouncer and dash into its right side to be flung past the next bouncer below, where you'll need to dash into its right side again to be flung near the feather. Dash into it and fly around the brambles ahead. Fall to the bouncer below, where you'll need to hit its right side to be flung across the screen to another bouncer, where you'll need to bounce off the top of the second bouncer to reach the right side of the third bouncer to be flung to Badeline. Hit her and exit right.

• Jump and dash to the first blue block, then jump to the bouncer and use it to keep you afloat while you wait for the blue block to go dark. Bounce off the upper-right part of the bouncer and dash up-right to enter the area above the platform part of the blue block to make it become usable. Jump off of it and dash up into the right side of the bouncer overhead to be flung to the next blue block. Jump to the last bouncer and use it to keep yourself alive as you wait for the pink block to go dark, then bounce off its right side to be flung to the heart. Dash into it and enjoy your achievement.

Timestamp: 1:55:22 & 2:14:08