One would think that to do this, you need to browse to the file with the file manager, but that doesn't give you the option to make a specific mp3 a ringtone.
You can do this, however... You need to do it from the built in Music app.
First is to go to and play the mp3 you would like to make your ringtone:
Now tap on the bottom left button of the phone (uhm, I don't know what to call that button actually haha! It's to the left of the home button in the middle on the phone) - this will bring up the options for the screen you're currently on, like so:
One of the options there is "Set as" - tap on that, which will give you this screen:
And voila, select the appropriate option, and you can now listen to Metallica whenever someone calls you :)
