How do you prepare your skirt steak for beef fajitas?
Cut the long skirt into squares, then slice thin against the grain - so you end up with 4-5" long thin pieces where the grain is running the short way (and therefore chewable).
I then marinade for 30-45 minutes, julienne red and green peppers and onions, and pan fry in my very hot wok.
In the summer, when I prefer to cook outside, I take the whole square pieces, marinade them, and grill over hardwood charcoal, then slice up the cooked pieces. I do the veg in a pan separately, using some of the marinade juices and then combine at the end.
The marinade I use is a doctored up McCormick's Fajitas seasoning mix in a packet. I've tried all the other ones - Old El Paso, etc., and none are as good as McCormick's. I doctor up with fresh garlic, fresh squeezed lime juice, and fresh ground cumin, then follow directions (oil, water).
I just wanted to mention that you really want a hot pan to cook with - and you might do the meat in batches unless your burners are virtually commercial grade (18,000+BTU). The worst thing you can do is "boil" the meat, rather than pan fry it. Squeeze the marinade out very well before dropping the meat in the hot pan and really get the meat sizzling. If your heat is too low, the meat will just sit in a pool of juice and boil. Even if you like your meat juicier, cook it dry, and put the marinade juices back into the meat/veg mixture at the end, rather than let the meat boil.
Leave your fajitas in one long strip for grilling, it makes for much easier handling. Be sure to trim all of the membrane from the meat before grilling; it makes for much more even grilling and a much better chew. That said, you do want to cut across grain as the previous poster suggested for a good chew.
Personally, I believe that all fajitas need is a sprinkle of S&P before grilling and a squeeze of lime after. Traditionally, that's all that's used here in Texas and down into Mexico.
Try to find the "outside cut" of skirt steak - much more tender to begin with
Marinate in lime juice and beer for 1 hour
Cook over hardwood and mesquite at high heat turning to get a good char on both sides
Let the skirt rest for a few minutes before slicing into strips
*Important: the grain on a skirt runs vertically across the short side not down the length. I slice the whole skirt into 5 inch sections and then slice these the long way for serving in tortillas.
Add grilled veggies and onions as you like. Guacamole goes well with them. Serve with fresh limes to squeeze over everything.