Hello Carol. Flank steak is a wonderful, flavorful cut, and you are entirely right that slicing it against the grain is the single most important step for ensuring it turns out tender rather than chewy.
To identify the grain easily, inspect the steak while it is still raw. Look for the long, parallel lines running across the surface of the meat, which look very similar to the grain lines in a plank of wood. These lines are the bundles of muscle fibers. On flank steak, these fibers are relatively thick and prominent. If you plan to apply a dark marinade or heavy dry rub that might obscure the surface, take a close look beforehand, or make a small nick on the corner of the steak with your knife perpendicular to the grain to serve as a guide later.
After cooking, allow the steak to rest on your cutting board for 8 to 10 minutes. Resting lets the muscle fibers relax and reabsorb moisture, which makes the meat juicier and much easier to slice cleanly without tearing.
When it is time to carve, locate your grain lines and position your knife at a direct 90-degree angle across them. Slicing across these lines shortens the muscle fibers dramatically, doing the tough work before the meat even reaches your plate. For the best possible texture, angle your chef's knife slightly at a 45-degree angle toward the board (cutting on a bias) and aim for thin slices, around a quarter-inch thick.
Take your time with smooth, single strokes rather than sawing back and forth. I hope your family dinner turns out fantastic.
