To cut flank steak for this recipe (or any stir fry), slice it in thin (about ¼" thick) strips against the grain. To find the grain, look for the direction that the muscle fibers run in the meat. You'll want to cut against (perpendicular) instead of parallel to these. For a more in-depth explanation, here's a quick video. And a tip to make slicing easier: freeze your beef for 30 minutes before cutting. This will harden it slightly.