In a large shallow container, combine the oil, vinegar, garlic, salt, and pepper. Add the steaks, turning to coat them well with the marinade. Cover and refrigerate from 12 to 24 hours.
Remove the steaks and pat them dry with pepper towels. Discard the remaining marinade.
While the steaks sit out and come to room temperature, preheat the grill to medium-high. Carefully oil the grill rack.
Season the steaks generously with salt and pepper on both sides. Grill the steaks, turning once about halfway through the cooking time for simple grill marks on each side. This should take 8 to 10 minutes total for rare (120 F internal temperature), 10 to 12 minutes for medium rare (125 F internal temperature), and 12 to 14 minutes (130 F for medium).
Remove the steaks from the grill to a cutting board and let sit for at least 5 minutes before cutting into them. Then slice the steaks across the grain and serve.
Notes
You can buy a 2-pound skirt steak and just cut it crosswise into two or three pieces, which is easier to handle on the grill.
Don't marinate the meat for more than 24 hours, or the acid in the vinegar can start to make the meat break down and turn mushy.
Make sure to only cook this steak to medium-rare or rare as it can be a bit tough when it's cooked beyond that level of doneness. Skirt steak is best cooked quickly over high heat, just until the outside is seared, so grilling is a perfect cooking method for skirt steak.
Make sure to cut the cooked steak against the grain to achieve the best texture and juiciness.