Welcome to Android Central! Use a file manager app (or connect the phone to your computer via USB and use Windows Explorer) to copy the music file to the Ringtones directory on Internal Storage. Then go to the Ringtone settings--when you select a tone, you should see the song as a choice.
Go to Settings, then Sound, then Ringtone. Scroll down to where it says "Add from Device Storage" and you can choose from the music on your phone. Finally figured it out today with my Note 7!