Sanyo8100: how to make an MP3 into a ringtone?

I have a website that can post a new ringtone to long enough to get it into my phone, the problem is, I can't figure out how to or what to convert the original MP3 format clip into.

I want a polyphonic clip, not a MIDI. How do I get an MP3 into the proper format for the Sanyo 8100 to play it as a ringtone? I know there's at least one site that will convert it for you for a fee, but I don't want that b/c (A) I don't want to pay the fee, and (B) I want to know how it works

Thanks,
d