Step 1: Pick a Song
I've always been a huge fan of Paul Simon and The Only Living Boy In New York is a personal favorite.
Step 2: Record The First Pass
The first thing I recorded was the final video clip and a one-take pass at the song to give me some structure to begin layering on top of. To get this first track I set up Garage Band to record using a microphone which was set up (off-screen) about 2 feet from me. At the same time I used Photo Booth to capture the video from the iMacs web cam. In the video I'm wearing headphones so that I can hear the click track.
Step 3: Overdub, Overdub, Overdub
At this point I had the one track to work with and began to lay additional guitar and vocal tracks over it, recording directly into the iMac using an M-Audio USB interface.
Step 4: Export the Final Mix and Sync the Video
After exporting the final mix from Garage Band into a high quality AAC file, I imported both the original video clip from Photo Booth and the audio track into iMovie HD. Once the import was complete, all that was left to do was to extract and remove the clip's original audio track, sync up the new audio and export to an H.264 Quicktime movie. I ended up forgetting to flip the source video so the final clip is still a "mirror image" due to the way Photo Booth captures video.
That's It?
I'm consistantly amazed at the quality of the iLife applications. I'm in no way an expert at Garage Band and this was literally the first time I've ever opened iMovie HD. Still, given a couple of hours and a low end Apple it's almost absurdly easy to complete a project like this. As a teenager I'd spend days recording and bouncing tracks on a 4-track cassette recorder, work that would literally take a fraction of the time on an out of the box iMac.