i stole it from fr0ng
1. Open iTunes and find the mp3 you want to convert to a ringtone
2. Right click the song and "Convert to AAC"
3. Right click on the new AAC file and click on "Show In Explorer"
4. Rename the extension from m4a to m4r (ringtone.m4a becomes ringtone.m4r)
5. Double click the new m4r file and it will be added to your ringtones library in iTunes
6. Go back and rename the m4r to an m4a file (ringtone.m4r becomes ringtone.m4a)
7. Play the file from your ringtones library. It will ask you to locate the file.
8. Once you find the file, play it and it will play again in iTunes.
9. Go to your iPhone tab and under ringtones, make sure the ringontes are checked for sync
10. Sync up and enjoy