Hey i'm back !
Just a quick tutorial, i was having such a crap time trying to figure this one out... so i hope it helps you!
remember to change the .m4a to .m4r
and delete the song in iTunes (but don't put in trash 'Keep the file') then drag the SongName.m4r to Tones :)
if you don't have tones on your sidebar, go to your iTunes Preferences and it's right there tick the box that says Tones :)