Have you made the 20-second version as an AAC file using iTunes on your computer or Mac? If so, try this:
How To Make Ringtones For An iPhone
- Assuming you've made an AAC file of the 20-second version (which I'm guessing you have, if you've managed to cut the song down to the length you'd like it to be), right-click on the song in iTunes and click "show in finder" (Mac) or "Open in Windows Explorer" (PC).
- If the file extension says ".m4a", rename the file and replace the ".m4a" with ".m4r". On a Mac, you can rename a file by right-clicking and selecting "Get info" from the menu. Save as ".m4r".
- Delete the shortened version from your iTunes library.
- Double-click the ".m4r" version to load it into your iTunes. It should come up as a ringtone.
- Sync your iPhone.
- The 20-second version of your song should appear as a ringtone on your iPhone. You can find this by going into "settings" and then "sounds".
Instructions may vary, depending on which version of the iPhone you're using. Good luck, and I hope this works for you!