How To: Reveal One Layer With Another

I saw a post online where someone was all confused about how to make something like this clip here, so I told him all you need is three layers:

• Top layer: someone shot on green screen, keyed, with posterize time.
• Middle layer: foreground art, using top layer as an alpha matte.
• Bottom layer: background art!

And then that guy was all like, "Isn't there a youtube tutorial that takes 45 minutes to give me that same information? Preferably one full of self-promotional nonsense at the beginning and end?" So...

I made one! Except it's only 96 seconds long. I'm sure I'll learn to pad them out as I go.