steps...
1) Right click song in itunes
2) click get info
3) click options
4) check both start time and end time box, and select the time you want for ringtone
6) click ok
7)now back in music, right click song
8) click create AAC version
9)New song version will appear underneath song.
10) drag new version to desktop
11) change extension to .M4R
12) drag new file to library in itunes.
Note make sure to delete the new version in itunes, and change back the original file so that it plays in full, otherwise it will only play the part you checked.