This requires you to get "gold stars" on an encore. This means getting more than a bit more after achieving 5 stars on a request. This can only be done while playing on expert difficulty. I did this after progressing through career mode enough to unlock the All Moderate Songs Set (in Los Angeles). This set of of 4 allows you to choose what songs you want to play, and they are all easy. After the 4th song, to my knowledge is always a request.
A few more tips if you are not great at rockband:
-play on bass
-always use your overdrive
-save overdrive from 4th song to have a full bar at start of encore
-make sure you have the highest multiplier for your instrument at end of 4th song
-you can restart the encore as many times as needed