You begin in the middle of a set of stairs with some crabs at the bottom. Well, paintings of crabs. Turn up the stairs and move towards the door that magically opens when you get near it. Step out onto the rooftop area around the little divider, and once you are on the opposite side of the divider that you come out onto the rooftop facing, literally just opposite of it to the doorway you went through to get onto the roof, you are done.