You COULD just find sheet music of the song, and use the rhythms with timers as a sort of silent metrinome, counting the measures, using that to tell you when a note should have been hit. Use the sheet music as a guide, and have it at such a tempo that it follows with the recording. Still seem easy? I tried making one of these once. it is NOT easy. I suppose you could use a different method, though. Maybe I'm just crazy.