Warhammer: Vermintide 2

There is one rune to unlock for each Back to Ubersreik level. Each one is pretty simple and can be done on recruit, but they each require a few steps to unlock. There's lots of small locations to visit for these, so you might want a video instead of a text guide (or in addition to it), but for those who want the text guide, here it is.

The Horn of Magnus

There are 5 bricks you must press throughout the first half or so of the mission. There are more than 5 bricks, so don't press any if you manage to find any that are not these!
-Shortly after the start of the mission, don't go up the stairs to the left into the first building; go right to find 3 boxes in a corner, the brick is behind the boxes.
-After the pig pens you will head up some stairs and cross a bridge that leads to an archway. To the left of the archway is a wagon, the brick is behind a box to the left of the wagon.
-Following the rightmost path after going through the archway, you'll find a cellar; go inside. Immediately to your right, you can find vines covering a small secret area; the brick is behind the beam in this area.
-After clearing the barrels section, stick to the left part of the open courtyard. In the back left corner, you can jump on a slanted box, then onto a stack of boxes, and finally, onto a small post sticking out that you can use to jump on the nearby roof. Jump around the corner to the left, then drop down; you'll find the brick and a tome if you wish to grab that.
-Heading up to Oliver's Inn, you'll find the bar inside. Behind the one of the corners on the counter of the bar is the final brick.

After getting the final brick, head backwards outside and go down the stairs. Head right as you're going down, and you should find a newly opened gate, which leads to Rune #1.

Garden of Morr

Progress through the level until you get to an large open area (you'll go through a fence to get here; this is pretty early on in the level) The objective should still read "Find the source of the Green Poison". Looking near where you would normally go (through the narrow and winding crypts), you should be able to spot two large statues, with a "Strange Skull" on a pedestal between them. Activating this skull will start an invisible timer in which you must light 4 unlit candles. You might want to try and find the spots for these first, as you can see them before you use the skull:

-Follow the alley immediately left of the skull, then take a left at the crossroads. You should find a small open area, with Candle #1 sitting on a small stone.
-From Candle #1, jump on the coffin and over the brick wall. Head towards the campfire to your right, and you should see Candle #2 along the way sitting on another stone.
-From Candle #2, there is a wagon ahead you can climb on top of. Jump over the fence and directly ahead from the wagon is a statue where Candle #3 sits.
-From Candle #3, head forwards back into the big open area. Towards the back there is a singular pillar standing out in the open. Candle #4 is on this pillar.

Once you hit Candle #4, a door behind the Strange Skull will open, and you'll find a Gargoyle head. Carry it most of the way through the mission, and be careful not to lose it. Once you have cleared the maze, enter the gate, and go down into the next area like you normally would. Shortly after entering, you will go up some stairs that curve up and to the left. At the top of these stairs, take a right and you'll see some windows. Drop out of the window to the right of the ladder, and you'll find a spot to throw the Gargoyle head. You should be able to find another small drop nearby (where the ladder would take you), and on this level a new path should be opened, leading to Rune #2.

Engines of War

Heading roughly straight (and slightly to the right) through the cave part of the mission, you should find a circle looking symbol on the wall. To the right of this symbol should be a path that lead back outside. You should see one lone tree with a rock behind it; climb on the rocks and keep jumping up until you find the camp where a torch is. Pick up the torch and take it with you for a bit.

Eventually the mission wants you to go through a log and progress through the barrels section. Don't do this, take your torch to the right of the log. You should find a stump you can jump on, and from the stump, jump on the nearby banner. From the banner, jump on the rocks to your left, and follow the path into a room. Throw the torch into the fireplace in this room by switching your weapon. In the room you should now clearly see 3 statues on the table. Take good notes of how each statue looks, as you will need to remember them for another puzzle.

After the barrels section, you'll go through some ruins. On the last set of stairs before the end of the ruins, you should see a tree branch to your right. Jump on the branch, and follow it back towards the tree. Then jump onto a stone pillar ahead, and finally onto a stone ledge to the right of the pillar (you should see a torch mounted on the pillar, anywhere to the left of this torch is where you're aiming for!). You should see many statues nearby, but only interact with the 3 you saw in the fireplace room. If you did this right, the door should open nearby, which leads to Rune #3.