Remove the silverskin and membranes from the skirt steak. If the steak is thick or uneven, lightly pound it under plastic to about ½ inch thickness for even cooking.
Combine the lime juice (citrus), beer, onion, garlic, cilantro, chili, oil, salt, oregano, cumin, black pepper, and optional soy sauce in a blender or food processor. Process until smooth. You can also use a molcajete to grind the ingredients.
Coat the steak well, cover, and refrigerate for 30-90 minutes. Skirt steak absorbs flavor quickly, so don’t over-marinate, especially if using pineapple juice. A sealable baggie works great for marinating.
Preheat your grill to very high heat and oil the grates. (A cast iron pan over high heat works great, too.)
Grill the arrachera. Remove the steak from the marinade and pat it dry. Grill for 2-3 minutes per side, until nicely charred and medium-rare (about 130-135°F).
Let the steak rest 5-10 minutes, then slice thinly against the grain. Serve in warm tortillas with onion, cilantro, lime, and grilled cebollitas if you'd like.