Two completely different tutorials on using green screen in your productions. One of them qualifies for inclusion as it describes the process in Final Cut Pro X, the other because it's informative and fun for a Friday.
The first tutorial is from Andrew Kramer at Video Copilot. His website is big on After Effects tutorials and presets, but we think the video is ideal Friday material.
The second tutorial comes from Dan Allen and covers green screen or chromakey in FCPX. Rather coincidentally, he is using one of the free green screen clips offered by Video Copilot to experiment with.
Dan is no stranger to FCP.co, we ran his series of Final Cut Pro X tutorials shortly after the new version was released.