If possible, first tenderize your flank steak with a Jaccard Meat Tenderizer. If you don't own one, it'll be fine, just not as tender.
Rub both sides of the meat with olive oil and then steak rub. If desired, youn can wrap the meat in plastive wrap and refrigerate for several hours to add more flavor.
Heat a well-lubricated grill to medium-hot and place flank steak on the grill. Brown evenly on both sides, but not past medium-rare to medium (about 140 - 145 degrees is perfect). Flank steak is actually a little less chewy when cooked a little beyond medium-rare. Rare flank steak can be a tad chewy.
Let meat rest for a few minutes and then slice thinly across the grain. This is really important! If you slice the meat along the grain, it will be tougher to the bite.