Flank steak is a very lean piece of meat. It needs to be cut super thin and across the grain to keep it tender. This process helps cut though those tough fibers lending it to a tender bite. Look at the flank steak and look to see what way the long muscle fibers are running. Then, line your knife up to cut perpendicular though the fibers and across the grain.  Do not cut parallel with the muscle fibers but across or perpendicular the muscle fibers. This method ensures a tender bite.