Here's the short answer: whisk together soy sauce, lime juice, neutral oil, brown sugar, garlic, chili powder, cumin, smoked paprika, and a pinch of kosher salt. Marinate skirt or flank steak for at least four hours. Cook it over the highest heat you can manage. That's the fajita marinade recipe. Everything below explains why it works, and what to do when you don't have four hours.