you might wanna do it like this:
1. Open itunes.
2. In your music library, right-click the song you want to convert to a ringtone.
3. click on "Show Info" (something like that )
4. go to "Options" (or "Extra"?).
5. Click "Begin-" and "Endtime".
6. choose the part of the song you wish to convert to a ringtone
(keep in mind: no longer then 30 seconds!!!)
7. Click Ok to confirm
8. Right-click again on the same song.
9. Click "Convert selection to AAC" (takes a little time)
10. Now you'll see the same song twice in your library.
11. Right-click the 30 second-song.
12. Click "Show in Explorer" (if you're on windows)
13. You"ll see a folder with a file in it with extension "m4a"
14. Change the ".m4a" extension to ".m4r".
15. Confirm the change.
16. In iTunes, right-click the folder called "Ringtones" and choose "File", "Import".
17. Go to the location of your ringtone that you just made and select it and click OK
18. When you are syncing iTunes, you can select the file and there you go, custom ringtones on your iPhone!