You can't keep your legs together like you do for regular pushups. The wider your legs are spread, the easier it is. Your arm is pretty much in the same spot as normal.
The triangle analogy is a good one. Head is the apex, and the feet are the bottom points.
Best way to practice them is to start doing them on your knees ("girly pushups"). This helps build the strength for when you go to your toes.
Don't forget to practice BOTH sides.