Sony Ericsson K790 / K800
How to make your own custom ringtones without having to flash your phone
If you have your phone locked and your carrier has put an mp3 restriction on it you cannot assign mp3's as ringtones, therefore you'll need to convert the mp3 to a wav file.
Best program I've found to do this was trusty iTunes...if you explore the preferences you'll notice you can any mp3 to a wav file, and iTunes also allows you to only play a certain portion of a song.
eg: you want the chorus of a song to be your ringtone so you specify in time (1:30 start) and the end (1:50)
I usually allow for only 20sec since my voicemail kicks in around 15sec or so, this also cuts down on size of the file since wav...