E51

Go to Nokia's website and download their PC Suite. [1]
Install software.
Get your phone connected, either via Bluetooth, Infrared and via a usb cable using the connect a phone wizard. [Under Settings > Add more phones if it doesn't automatically run first time.]
To get the mp3 to your phone, drag the mp3 to the PC suite main window to the picture of you phone where it says drag and drop content here.
Select do not convert on the pop up window
A pop up will tell you when the transfer is done, it will then tell you where the file is on the phone [E:\ being a memory card if you have one.].
On the phone go to Office > File Manager and go the folder indicated in the pop up. If for example the pop up said E:\Music, scroll left to go the memory card, the scroll down to the music folder.
Open folder and find your .mp3 file.
Select the file. It should start playing. [Press select again to pause.]
Go to Options > Use Tone > Set as ringing tone > Select profiles to use it for > Ok
Et viola your done. :)