Band Hero

Choose Career or Quickplay from the main menu and on the next screen select Choose Rocker. Scroll until you find 'Avatar' and select it. Now play through any song and you'll get the achievement once the song has finished.

To get this achievement, simply play a song as your Xbox Avatar and after completing it, you'll get it

If you want to get this quickly you can play a created song (Details on how to make one and get an achievement along the way - Solution for Studio Time In Band Hero)

You can unlock this in any game mode (Quickplay, Career, Online or Competitive) in the game lobbies scroll down to the Rocker menu, Avatar is the option at the bottom of the list, select you avatar and complete a song and the achievement is yours.

Here's a video using a short custom song