1. Select the song you want to convert to a ringtone in iTunes. Right-click and choose 'Get Info'. Click the Options tab. Set the Start and Stop time for the sample (no more than 30 seconds). Click OK.
2. Right-click the song in iTunes again and select 'Create AAC Version'. This will create a sample of the original track in iTunes to the length you set in step 1.
3. You can now set the original song's start / stop time back to how it was (undo step 1).
4. Right-click the sample track in iTunes and select 'Show in Windows Explorer'.
5. Drag the new file to your Desktop (or any other folder).
6. Delete the sampled track from iTunes (right-click, delete). The file can be moved
to the Recycle Bin.
7. Browse to the copy of the track you made in step 5. Right-click and rename. Change the extension from .m4a to .m4r.
8. Import the file back into iTunes (File -> Add File to Library) and it will be added to your library as a Ringtone.
That's it!