You can just use Itunes.
1. Select any MP3, right click and Get info
2. Go to Options and the select Start Time and Stop Time of your ringer. (No more than 30secs)
3. Click Ok and go back to the main Itunes windows with your song and convert it to Apple lossless (m4a).
4. Click and drag m4a file to desktop, then delete the one in Itunes.
5. Rename from ringer.m4a to ringer.M4R
6. Drag to Itunes and it will load on the Ringers folder.
7. Sync and you are done.