This is a video showing the glitch and how it works:
1: use 1st person view
2: go to a building's wall or any other flat surface you want to look through
3: go to kneel stance (makes the glitch a bit easier to apply)
4: push your shoulder against that wall (preferably the left shoulder)
5: use free look to look at the wall
6: lean towards the wall and look through it
7 (optional): use the zoom function if leaning towards the wall alone is not enough for being able to look through it.