Turn on the Bluetooth on your phone and go into the Bluetooth utility on your computer. Set your phone to be discoverable and then connect to it from the computer. Click to send a file, and send the sound file that you want to use as a ringtone. The phone will receive it, and you can save it and use it as a ringtone. If these instructions aren't detailed enough, I can open mine up and see what to do.
I can't find any "save" option once i get it on my phone. I'm on Verizon if that changes anything.