Put all the ingredients for the marinade in the food processor and pulse until blended.
Scrap down the sides of the processor with a plastic spatula, then add a bit of water to thin the marinade and run the processor again. You should have 1 cup of marinade.
Place the skirt steak in a large zip top bag and pour the marinade on top. Mush the liquid around, making sure to get it on both sides of the steaks.
Seal the bag and place it in the fridge overnight or up to 24 hours. ( Several hours was plenty. )
When ready to cook, take the meat out 30 minutes before cooking so it can warm up to room temperature.