Pick the song, highlight it -- right click -- get info -- options -- choose the best time of that song example start time = 0:20 end time would be 0:50 -- click ok
Make sure you have the convert set to mp3 format. Right click the same song again -- create mp3 version (if you do not have this option then you need to change it) GO TO -- EDIT -- Preferences -- General -- import settings -- Import using -- choose from the box and pick mp3 then click ok.
Now you should have the create mp3 version. itunes will then create a 30 sec of that song.
Check your music library and click time so you can see the duration.
NOW drag that song to your desktop. then just move it later to your ringtone folder..