download the mp3 you want,hook up the phone to the sony docking station,make sure its connected,open up the folder the mp3 is in,open up the p900 by using the shortcut on the desktop (mine is called myp900) open up memory card or phone memory depending on where you want to put it,then open up the audio file and drop your mp3 into it

now take the phone off the docking station open the flip,select control panel,sounds and alerts,then choose find sound on the drop down file next to RING VOICE,select find sound then search the phone for the song.also because of the limitations of only being able to playthe mp3 for 20 seconds as a ring toneyou might wanna get a mp3 splitter and take out the best part of the song.