Once you exit the bathroom stall, head to the left wall (to the left of the urinals). While facing the wall, make your way to the right and all the way around the room and you will be able to interact with several tiles that will fall off. One will reveal a small safe.
Turn off the lights with the switch next to the door and press
to turn on your phone's flashlight. Make your way to the urinals and flush them one at a time from left to right, noting the direction of the red arrow in each. Then interact with the safe and input the five directions from the urinals with the d-pad. Be careful with your inputs as this achievement requires not making a mistake and opening the safe the first time you interact with it.
If you make a mistake, you'll have to exit to title and restart from the previous save.