Here is a short tutorial that points out the most important steps

Hardware:

Software:

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 8: Drag the first iMovie video file into video track 2 and sync it to the GarageBand file. Just play both tracks and move the video file until it is in sync with the GarageBand mix down. Once the first video is in sync lock it and mute its audio.
Step 9: Repeat Step 8 until all videos are in sync.
Step 10: Unlock all video tracks, size and crop all individual tracks (in Final Cut Express 4: double-click the track, go to Motion, and size and crop), and position them the way you want.

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