This is how I did it.
You need 2 programs.
1. Ringroid: It allows you to cut songs, and make it however, long or short you want or choose whatever part of the song you want. You obviously first have to transfer music from your computer to your phone, and you do that through the USB, dragging whichever music file to the Android phone and syncing.
2. Rings Extended: Once you install, it does not launch or anything. But if you go to settings;notifications, or whichever part you want to set your music as a ringtone, if you select it, they will give you two options, android system, or rings extended, and you go to rings extended, and check whatever song you want.
There you have it. Hope this helps.