Kao the Kangaroo

There are quite a few collectibles in this game, so it's important to know what's required:
 

• KAO Letters - Every level has the letters K-A-O hidden throughout, in sequential order. They're the same color as coins but a bit bigger
• Heart Fragments - Every level has at least one, but upwards of three, Heart Fragments to collect. Collecting four total gives you and extra heart
• Eternal Wells - Most levels have two Eternal Wells. These act like challenge rooms. Note that all that's required is to beat them. It doesn't matter how many coins you collect or if you collect any gems
• Runes - These are required for progression, and out of the 50 in the game, you need 48 to access the final boss, so you'll be unavoidably getting almost all of these anyway. The ones in levels are always right on your path so you don't miss them. The ones in hub worlds are the only ones you may need to hunt down

And what's NOT required:
 

• Gems - After finishing the game and doing seemingly everything, I still have absolutely no idea what these do. I never used a single one, so feel free to ignore them completely
• Scrolls - Here I'm referring to the scrolls that are tracked for each level on the pause screen. Each level and hub has one or more scrolls hidden somewhere to collect. These are not required for any achievements. The scrolls that are required for Find’em All (90G) are dropped by enemies and aren't the ones tracked on the pause screen

Note that, of the required items, Heart Fragments are the only one not tracked on the pause screen. In fact, I couldn't find them tracked anywhere. This means that if you miss one, you unfortunately have no way of knowing which level you missed them in. Also be aware that, in addition to the Heart Fragments you need to collect in levels, the shop in each hub world sells two Heart Fragments, so make sure to buy the two in each hub world when you get there. 

The good news about tracking is that all collectibles in the game save as soon as you pick them up. This means if you grab one and die, it stays collected. This also means that if you miss anything and need to replay a level to get it, you can simply grab it and then quit back to the hub, and it'll stay collected. 

In terms of a guide, I'm the type of person that prefers a text guide over constantly watching a video. The best text guide I've found is by PowerPyx, HERE. If you prefer a video guide, refer to THIS PLAYLIST. Full credit to the creator of that content.