Go into Forge and select the map Sandtrap.

When you get there get in a Warthog. There's a place in those pyramid things and the two plaforms that are side by side.

Go there in the Warthog, go up againt the wall to where the driver side is all the way against the wall.

Then get out and if you look at him through either the theater or another player you look like you're dancin on a Warthog.