This again can be done in any game mode as soon as you enable Fitness mode in the Fitness menu. Each time you finish a song, look for the message in the upper right corner of the screen indicating how many calories you've burned so far. The message usually pops up in 100-calorie increments for your running total. It seems to be approximately 12 calories per song on easy, but should come naturally from playing all of the songs in Perform It mode and the Crew Challenges on the various difficulties, especially attempting to learn and 5-star every song on hard. To check your progress, go into Fitness mode and look at your overall stats. This will likely be one of the last achievements you unlock.