A song counts as "fully evolved" you need to get all 4 gold stars: Acting, Music, Singing and Dancing on "Best Friend's Brother" (5th song) and the achievement is yours. Gold stars are unlocked by getting at least 600.000 points during a performance.
EDIT: Thanks to Sashamorning
Sometimes if you step out of the frame it doesn't recognize that it's you that came back. I had to restart the game and sing the song over. If you already have 3 stars, it should pop on the next play.