Step one: The game master (GM) initiates play by hiding her eyes. This renders her effectively invisible; other players must indicate this by saying things like “where’s Sydney? Where’d Sydney go? Have you seen Sydney?” [1] If the GM is peeking between her fingers, it may be helpful for the other players to visibly search for the GM under bowls, inside the toaster, and in their breast pockets.
[1] It is probably for the best if you substitute the name of your particular GM for “Sydney” when you attempt playing Peek-a-boo at home. Alternatively, you could convince your GM’s parents to legally change their child’s name to “Sydney.”
“Boo!” as an RPG? A wonderful post at Alas, a blog.