How to add music i've bought on my iphone 5 as a ringtone?

So i bought a bit of music from an album and i want to add it as my ringtone, is there anyway to do it?

Buy an app called ringtones, edit which part of the song you want. Then plug it into your computer, and extract the mp3 from the "documents and data" (or something like that) from the app, save it, and sync it as a tone