1. Attach the phone via USB to a computer.
2. Drop down the notification bar and tap "Select to copy files to/from your computer", then "Mount".
3. Create a folder on your SDCard called "Media"
4. Create 3 folders under the new "Media" folder, each called "Ringtones", "Notifications", and "Alarms".
5. Place the mp3 you want to use into the corresponding folder. (i.e. If I want to use an mp3 for an SMS Alert, I would put it under "Notifications". Or as a ringtone when someone calls me, under Ringtones, or as an alarm sound, under "Alarms", etc.)
6. Drop down the notification bar on the phone again and select "Turn off USB Storage", then "Turn off".
7. Wait for it to prepare the SDCard, then give it a few minutes to refresh all of the tones.
8. You're done.
Note: All folders are without quotes.