In order to unlock this you need to get an "S" rating (the one above "A") for all three songs in a standard three song set in superstar (the top option in the main menu) mode. You won't get this for getting three "S" ratings in a row across two different sets, or for getting "S" ratings in any of the bonus sets.
You don't need to pull off a full combo to get an "S" but you can't have too many misses. I think the longer your longest combo within a song, the better your chances of getting an "S" are.
If you don't know the song, don't worry -- the pitch detection in this game is pretty much garbage. Just singing a straight single tone or even putting the microphone in front of a loud fan (especially the latter) is enough to to trigger a full combo in most songs.