A sim­ple app ought to do it.

Down­load iRinger. It used to be a Win­dows app, but is now avail­able for Mac OSX!

A very intu­itive appli­ca­tion. Select a song (mp3 for exam­ple), it clips it and shows you the ringtone-worthy seg­ment of it, you basi­cally export it as a ".m4r" file.

When done, just open the file and iTunes will auto­mat­i­cally open it. Done.