Here is a short tutorial that points out the most important steps
Part A – audio & video recording
Step 1: After deciding and practicing the different parts of the song you will be recording, start GarageBand and create a new music project.
Step 2: Once you have set everything ready for recording, open iMovie and start the webcam (iMovie will ask you to name the iMovie-event). Now, start filming the first video track and go to GarageBand to start recording the audio track for the video track you are just filming right now.
Step 3: Repeat Step 2 for every additional track — Again: Record audio on GarageBand, film video on iMovie.
Step 4: Mix your song (audio) on GarageBand.
Part B – video editing
Step 5: Start Final Cut Express 4, make a new project, and save it with a name of your choice.
Step 6: Import all video files you want to use for your video from your iMovie event. Also import the GarageBand audio file.
Step 7: Drag the GarageBand file into the audio track and lock it.
Step 11: Render the whole video and export it. Your video is ready to be online !
Edit 07/11 : Here’s a very well done video tutorial which is much accurate… thanks to Christopher Kevin
Edit 07/11 : Here is a tutorial for the editing in Final Cut Pro X… Thanks to Isaac Kolding