Easy.
First, cut the desired song to length using an audio editing software (15 to 20 seconds). Save in MP3 format.
Second, connect your phone via usb and tell the phone to "mount" by selecting from the drop down notification drawer.
Third, create a folder on the root of your micro sd card labled "ringtones", all lowercase.
Fourth, drag and drop music clip into ringtone folder.
Presto, the ringtone will show up under ringtones when you browse through them.
There are several free music editing programs for free. Just google it and pick the one you like. Good luck and Merry Christmas.