Hi, I was just trying out the guide using garage band, and found difficulty exporting the file as a M4a to then change the extension to mra or something, when I came across the most easy solution for a mac user. there is a a ringtone export feature in Garage band! yeah just click it, it says you have to have a song under 40 seconds and if you haven't already adjust it so it loops then hey presto you can export a ringtone directly to itunes.
It works I just tried it out, now I am going to make custom ringtones for all my favorite contacts, to be honest the ringtones that come with the iphone are pretty boring. I hope this helped.
note: In Garage band just drag the song file from itunes cut down the length and click export, then ringtone to itunes. Make sure you have the latest update. But Garage band does not accept protected AAC Files so you cant use your itunes store purchases.