Can somebody tell me how I could send an audio file to my Samsung R540 (Telus) and have it considered a ringtone? Also what would be the audio file properties requirements?

Up to now, I'm able to put .3G2 and .mp3 on my SD card and play them on the phone. They're under 30 seconds and under 512 Kb. But I can't figure out how to make them "ringtones".