Microsoft’s update to Windows Phone, Mango, brings the ability to create your own custom ringtones from MP3’s etc – which is great.
It’s not quite as easy as Android, where you just long hold an MP3 and “set as ringtone” so here’s a little guide on a good way to do it…via a great Windows Phone site:
They recommend doing it the way I’d do it anyway so I find this the best route…however you may have different programs you like to use etc.
Head over to the link above and then get started with some custom ringtone action 
ps: If you’ve not got Mango yet – check this post out:
Cloudy with a chance of Licensing 