Open GarageBand (Macintosh HD > Applications > GarageBand). The project containing your music may load automatically (if it was the last project that you worked on). If it does not load, click File > Open on your menu bar and select the project from the navigation menu.
Add new tracks to your music mix. If you still have more to record, or if you have yet to record anything, create a new track at any time by clicking the "New Track" button. It appears in the lower left corner of your screen as a large plus sign.
Record onto your new track. When the new track appears (it looks like a horizontal bar on the left side of your screen), click it once to select it and then click the red "Record" button at the bottom of your project window. Do this for as many layers of music as you need to add.
Mix your tracks. After you have finished recording every piece of your mix, add any final effects before saving your mix as a song file. Click the "Track Editor" button (resembling a pair of scissors, in the lower left corner) to change the pitch of a selected track or delete portions of a selected track. Click the "Track Info" button (resembling a letter "i") to apply specific musical effects to a selected track.
Click "Share" on your menu bar. From the drop down menu, select "Send Song to iTunes." As of 2011, GarageBand will only export song files to iTunes, but since all computers running Mac OS X have iTunes by default, this should not be a problem. On the pop-up menu, enter the information about your song (such as title and artist name) in the designated fields and choose a compression type. Apple recommends the AAC encoder, because you can then easily convert to MP3 format in iTunes. Click the "Share" button."