To gold star a song, you need to be playing on expert and get around 1.4X the five star score(it has been made easier since RB1). When you're playing you should see a star meter under your score, a circle goes around the star as you get more points. As you fill up the circle, the star becomes whole and it moves onto the next one. Once you get 5 stars, and you hit the Gold Star score, your stars turn gold. Do it on Expert vocals with the same technique you did to get Flawless Singing, the Beastie Boys song, just use starpower whenever you get it.