Everything I know about using multitracking with video I learned from Linda Louden - credit where credit is due - she's the best and is great about sharing her knowledge. I use a really old macbook and I have a samson go mic (cheap usb mic) which is the same one Linda uses. Here is exactly how she showed me how this all works. (If you are using a PC, I am of no help)
Per Linda:
Multitrack
#1. record your song in iMovie. Doing this first helps music tracks line up correctly to your movie and mouth movement after using GB and importing back to iMovie. If you are going to add a title or transitions, now is the time to do that too because that adds seconds to the length. It is possible to do it later once your familiar with how best to line up music but its a good habit to get into cuz it saves steps.
#2. Share the movie to desktop.
#3. Open GB and Name the new project. Drag the movie into GB.
#4. After Movie has fully loaded into GB you have vocal settings to work with. On my version in the lower right you have the three buttons for media, info and loops. click on the "i" info button. I almost always use vocals- Live Performance and then click on the edit tab. Use the Compressor to can change the "gain" higher or lower if you need to. Fix these things while the music is playing to see what suits you. I always change to a lower the reverb. Usually somewhere in the 20's depending on what suits the song.
#5. Adding new tracks. Highlight the track to record your new instrument. You have to use earbuds or a head phones so it only picks up the instrument used. If you try without them it will pick up the music track too and you don't want that. Do this step for every instrument used. Sometimes I use a Bari, cajon, tambourine or a shaker and they all have a separate track of their own. You can cut and splice the track by moving the bar to a the specific spot and use "command t" to split , move the bar to a new spot, command t, highlight what you don't want and click delete. If you make an accidental delete immediately go up to edit and chose undo.
#6. Use #4 for each track.
#7. When everything is as you like, its time to close out. Click the red button to close. A message will come up to save. Chose Yes and chose Yes to save in ilife.
Now Back in iMovie!
On the left you will find Content Library. Chose GarageBand and it will load up your GB library. Find your project. Click on the plus sign and it will import under your movie and should be lined up exactly! Lower the sound on the movie track and adjust the imported track volume. Thats it! Now your ready to import back to the desktop and into youtube
Now if you want to show yourself playing a different instrument in parts of the movie here is what I do.
Click on import and turn on your camera.
Go into GB and with your earbuds on play along with the first original movie track to capture you in action. For Zipadeedoodah I filmed the slide whistle from the beginning to end of the song. In iMovie I drug just the part I wanted onto of the other movie. Made adjustments to lined it up and then imported that to GB and followed the GB directions tweaking out any bad notes, or rerecord it all together. Then in iMovie delete the old imported track if there is one and click the plus sign for the new track with the whistle.
If your not very familiar with iMovie then this might not be real clear. Let me know and I will break it down better for you. There are different settings for dropping in new film like "side by side, blue screen, pic in pic etc". Make yourself familiar with those if your not already and this all becomes easy stuff, but can become very time consuming.