Use the following form to upload an MP3. Your phone only has 40mb of internal memory, so I've limited the size of the file to a maximum of 1 mb. Name the tune in the next box (use 14-16 characters if you don't want the name to scroll when browsing your ringers). Then pop in the internal song number (what's that?).
After it's done, you'll receive the three necessary files to put in to your phone.