In order to obtain this achievement, you'll need to play through E1M1's tutorial until you reach this elevator.



Once up top, turn around and face these signs where you'll need to wall run to the chained fences on your left and there's where the QR code will be.






In order to obtain this achievement, you'll need to play through E1M2 up to the point where you obtain the shotgun.

Clear out the enemies in the same area and you'll see a stage with red flashing lights which you will need to climb up to get to the QR code.



Climb up to the top of the stage and the QR code will be there on the ground.





In order to obtain this achievement, you'll need to play through E1M3 up to the point where you reach the rooftops.

Once you find these two billboards, wall run along the right (lower) one to the building across.








In order to obtain this achievement, you'll need to play through E1M4 until you get to this curved wall where there will be a red container beside it.



Once you're here, wall run to the rooftop which is straight ahead of you.

When you get to that rooftop, the QR code will be inside of a small hut on the ground.





In order to obtain this achievement, you'll need to play through E1M5 until you get to the boat ride.

Right after the boat ride, look up above the boat and you'll see this billboard you will need to wall run to get to the top of it (shotgun jump required).



Once you're on top of the billboard, go to the ledge straight ahead of it and you'll find the QR code there.






In order to obtain this achievement, you'll need to play through E2M1 until you enter the first main building and enter the big open area.



Make your way across the room and you'll see a small tower-like thing and on top of it you'll find the QR code.






In order to obtain this achievement, you'll need to play through E2M2 until you reach the first big room (after the 4 berserkers).



The next thing you need to do is climb up to this platform.



Once you're on it, find this platform and that's where you'll find the QR code.

In order to obtain this achievement, you'll need to play through E2M3 until you get to this train station part.



Once there, clear out the enemies and activate the train as the game tells you to do.



After clearing out every enemy, stay in the same room and go over to the raised train platform on the other side of it.



Once you're up there, shotgun jump on top of the broken orange screens across from you.



You will find the QR code on top of one of the screens.



In order to obtain this achievement, you'll need to play through E2M4 until reach this first big open area of the level.



Once you're here, go to the main entrance gate from where you came from and turn to the left, you'll see some pipes there which you will need to shotgun jump to.



When you're on the pipes, keep walking until you reach the end and you'll find the QR code there.



In order to obtain this achievement, you'll need to play through E3M1 until you reach this wall jumping area with the red barrier.



From there, turn to your left until you see this opening down beneath the ground you're standing on.



From there head down and you'll find the QR code there.




In order to obtain this achievement, you'll need to play through E3M2 until you reach the part where the man talks to you in person and then start shotgun jumping on top of one of the two buildings.



When you're on top of one of them, you'll see many different platforms on the level's walls. Go through each one until you find the QR code on one of them.

In order to get this achievement, you'll need to start E1M2 and keep going through the level until you reach the part where you obtain the shotgun.

After clearing out the enemies in the same area, you'll notice that there is a stage with red flashing lights.



Go up on the stage and you'll see a backdoor and you'll see stairs that lead down to under the stage, which is where you will see an activation lever.



Once you turn on the lever, go back to the stage and come to this record mixer where you will need to pinpoint this exact same circle on it to get the "Use" button.



After you press the use button, the achievement will unlock.

This took a longer than I expected it to be lol but I think it was worth it in the end.

Notice: The screenshots for the MOTHER QR code achievements are my own screenshots but I also wanted to give credits to Citizen for creating the original guide for the MOTHER QR codes and their locations since I definitely wouldn't have been able to find them on my own.