Skirt steak is perfect for fajitas and absolutely delicious, but there a few things to know to insure tenderness. First, remove the membrane if it hasn’t already been done, you’ll see it if it’s there, it looks like transparent skin, just use your fingers and peel if off. Make sure to marinate at least 24 hours or up to 48 hours. Don’t cook it past medium rare and cook it over very intense heat and always cut across the grain and on the diagonal.