How to Convert iTunes MP3 to Ringtone

1. Open iTunes and find the MP3 you want to convert. Right-click the song and choose "Get Info".
2. Click the Options tab. Check the Start Time and Stop Time boxes, then enter times for the ringtone (maximum length for a ringtone is 30 seconds).
3. Click OK, then right-click the song again and choose "Create AAC Version". A new 30-second version of the song will be created in music library.
4. Right-click the new 30-second version song and choose "Show in Windows Explorer". Copy the converted m4a song to a temporary folder.
5. Change the file extension from .m4a to .m4r. If you cannot change file extension, open Folder Options from Control Panel, click View tab, uncheck the "Hide extensions for known file types"
6. Double-click it (or drag into iTunes) and it immediately gets added to iTunes' ringtone library.
7. Delete the 30-second version song from  music library and undo the Start Time/Stop Time changes to the original MP3.
8. Finally, sync your iPhone. Go to ringtone settings and you should able to select your new ringtone!