I don't want to steal Steve's thunder, but just the other day I made my own video about this very topic. The basics are really easy.
I used Logic 7.1, but the procedure is basically exactly the same in 8.
I'm sure Steve would go into more depth and the gotchas involved, but just in terms of making them, this video might have you covered.
[url]http://www.chellman.org/audio/how_to_make_green_apple_loops.html[/url]