I take the stems, listen through them one by one, hearing absolutely everything that goes on in the piece of music and sometimes I find a lot of stuff I didn't notice were in the song at all, like the guitar solo in MCR's SING, it's buried in the mix but with the Rock Band 3 dlc, it's in plain earshot. But with most songs I find parts that others might not have heard in the original (like the little keys in Slipknot's Psychosocial) and restructure the song to highlight the easily missed things.