- If you're using a song from your iTunes library, just open GarageBand, open iTunes, and simply drag and drop the song from iTunes to a new track in GarageBand.
- Now click the Cycle Region button pictured above, which will activate the region loop tool.
- Now adjust the length of the region to the section of the song you want to export to iTunes as a ringtone (40 seconds long or less), again as pictured above.
- Finally, when you've got everything set the way you want it, just go to the menu bar and click Share -> Send Ringtone to iTunes. GarageBand will automatically convert the song to the proper format and sync it to your iTunes library as a Ringtone.
Simple, no? Likewise, if you prefer to create ringtones of your own music, just build your own tracks in GarageBand and repeat steps two through four.