Here's how it's done, if I remember correctly. It's been a while though, haha.
1. Aquire some kind of audio editing softoware. I use Cakewalk Sonar software, but that's expensive and I only bought it for my recording studio. Most people tend to use Audacity. Google it, I can't link you to it yet (10 post rule haha).
2. Import the mp3 you wish to ringtoneify into the software, and cut it down to like 30 seconds or so. Any longer and I hear you risk crashing the phone. I dunno, but it's not like the phone rings for longer than that anyway.
3. Save the clipped-down sound as a 22KHz, 32kbps mp3 file. If audacity can't do that, try downloading another audio converter. I've found Easy CD-DA Extractor works well (google it, haha).
4. Go to where the file is saved, and rename it from file.mp3 (or whatever) to file.mid
5. Send it to your phone in bitpim.