Right after your bathroom escapades, you'll walk into a main hall type thing, in there is a solider stood next to a whiteboard. He'll ask if you want to contribute anything to the plan. Take him up n his offer and select the whiteboard.
Whilst drawing the achievement will unlock. Draw whatever you feel like, doesn't matter to the achievement or the soldier who keeps claiming it's genius.
Note: You do have to draw something, erasing all his hard work is insufficient