How can I set a purchased song as a ringtone on my iPhone?

I bought a song from the iTunes Store. I wanted to make it the ringtone for my girlfriend when she calls me. When I edited her contact info, the song was not listed as an option for a ringtone. How do I get it on the list so I can set it as her ringtone for when she calls me?

[Re-Titled by Moderator]