1. Right click on the song (in iTunes) and click "Get Info"
2. Options Tab
3. Edit the "Start Time" and "End Time" to where you want the ringtone to start and stop
4. (Click OK, but you knew that)
5. Right click on the song and select "Create AAC version" (if it doesn't say that go to 5a, if it does go to 6)
5a. Edit -> Preferences -> Import Settings (middle of the box) -> (Import Using AAC Encoder). Go back to 5.
6. Once you've done that you should have a duplicate of the song, but just the part you enclosed back in step 3.
7. View your desktop (or w/e) and iTunes at the same time, click and drag the new song to the desktop
8. Rename the file extension from m4a to m4r.
9. Double click on the "Ring" file, it should open in iTunes.
10. Sync your iphone, on the ringtones tab... etc.