Make the ringtone a less than 30 seconds long .m4r file (it is a renamed m4a file), put it in "Tones" in iTunes, and sync with the phone. Don't forget to enable sync'ing Tones when the phone is connected.
- Figure out the starting and ending time of the sound snippet by looking at the video in youtube.
- Get the youtube URL, i.e. http://youtube.com/watch?v=Rqz_JyAUZyg