How can you get a completely flat belly in a week?

Very unlikely: There's no such thing as natural spot fat loss. You need to lose weight in general & a certain % 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. Engage in regular aerobic exercise.