How do you lose belly flat when you are already skinny?

Fat loss requires wt: Loss. Spot fat loss does not occur naturally. By losing weight in general a certain amt will be abdominal fat. Eat healthfully ; be physically active. Calories burned must > calories consumed. To tighten ab's do crunches, reverse crunches, exercise ball crunches, pelvic thrusts, lying leg raises, side bridge ; “bicycling” exercise on your back. Get regular aerobic exercise.