Grease Dance

If you want to do this in even an easier method, just turn up your speakers and place your microphone right up to it. Elvis will sing the whole song for you. I did this with Hound Dog and got 90% without even singing.

Like the description says, you need to hit 75% of the phrases 'perfect'. Like most music and rhythm games, at the end of a phrase you are rated, and, in this game, perfect is the highest rating given. You'll need to hit at least 3 out of every 4 phrases with a 'perfect' (shows up on screen in green writing) for this to unlock.

Personally, I got this my first try on "Hound Dog". The song is an iconic Elvis song, and is listed on the 'Easy' difficulty page.