Often, for assignment submissions or performances, it's desirable to sync audio recorded in GarageBand with a separately recorded video. To do this, you will need to record the audio of your performance in GarageBand while simultaneously recording the video using a video recording application, such as QuickTime or a smartphone, then synchronize them afterwards. In order to do this, you can take the following steps:

1. Start recording the video, then the audio in GarageBand. 
2. Begin with a sharp, loud sound (like a clap). If you're recording an electric instrument, a sharp, hard downstroke on the strings or keys can be used. This will be used for syncing the audio and video later.
3. Once you're done recording, export your video and open it in GarageBand by going to File > Movie > Open Movie. This will import the video, as well as the video's audio, into GarageBand. 
4. Synchronize the video and audio by dragging the GarageBand audio so its transient sound from Step 1 (referred to as the sync sound from here on) is aligned with video audio's sync sound from Step 1. Note that you cannot move a video in GarageBand, so this must be done by moving the audio recorded in GarageBand.
5. Once aligned, mute the video's audio by pressing the track's mute button. The session should now contain the recorded GarageBand audio synced to your video.
6. Once you are satisfied with your video, you can export it by going to File > Movie > Export Audio to Movie.
7. Select a name, location, and resolution for your video, then click Save. Note that Full Quality and 1080p will be the best resolutions for Berklee Online assignments.