After you’ve gotten all six items for the pedestal room and turned the pressure valve, you’ll want to return to the pedestal room in order to pull the main power switch. After you’ve done this, leave the room and go straight down the hall to the right where Boris is strapped down.
To the right of him at the far wall is a poster.
If you’ve done it correctly, you can walk straight through the poster and see a stick figure of theMeatly, which will give you the achievement.
