Score skirt/flank steak, being careful not to cut all the way through the steak. If your skirt steak is a long strip, you will want to score it multiple times horizontally or on a slight diagonal.
Whisk the marinade ingredients together in a large glass measuring cup or small bowl. Toss into a freezer size Ziploc bag along with the steak. Get rid of any air, and seal. Massage marinade into the steak with your hands through the outside of the bag. Marinate in the refrigerator for 2 to 12 hours.
Let the steaks sit at room temperature for up to 30 minutes before cooking. Preheat grill to high heat, (550 degrees F with the lid closed). Clean, and grease grill grates with vegetable oil or olive oil spray.
Grill steaks for 2-5 minutes per side depending on thickness, or until an internal thermometer reads 130-135 degrees F for medium-rare. The tapered end of the outside skirt will grill the most quickly. Grilling times will vary depending on thickness of your steak. Use a meat thermometer to ensure you don't overcook the steak
Rest the steak for 5-10 minutes.
Slice the steak AGAINST the horizontal score marks. You want to cut across the LONG side of the strip steak. It's counter-intuitive to what you might think. If you have a long strip, cut it into 2 or 3 sections. Turn each section 90 degrees, and slice.