Lay it out and split
Lay the steak fat-side up, then split it down the middle cutting along the very conspicuous grain to get it into more easy-to-manage one-foot-long pieces.
Slide under the fat
Slide your blade underneath some of the exposed fat, being careful not to go too deep. The meat underneath that fat has a rough, ridged surface, so it's very easy to accidentally cut some of it off. Better to take the fat off in thin layers so that you don't accidentally over-trim.
Work your blade along the steak parallel to the cutting board, always making sure to cut away from your hand. Or, if you'd prefer, cut toward your hand...a single accident should set you straight for life.
Don't trim too much!
Fat = flavor and juiciness, so don't worry about trimming too much! You want to trim just enough that most of the meat surface is exposed, but there's no need to work your knife in between the meat fibers to get at the deeply situated fat striations. They'll help the meat cook better.
Flip and pull
Even though most of the tough membrane in an inside skirt will have been removed for you, some remnants may remain. Flip the skirt steak over to its non-fatty side and pinch the surface of the meat to locate the membrane.
Peel it off
Once you've got the membrane, it should be easy to peel off in large swaths. As with the fat, there's no need to go crazy here—just get most of it, and the rest will melt away as the steak cooks. With skirt steak, undertrimming is always better than overtrimming.
Ready to cook
One skirt steak, ready to be grilled.
Unfold your steaks
Whole skirt steaks are about two feet long and usually come rolled up from the butcher. You'll want to unroll them on a large cutting board, your sharp boning or chef's knife at the ready.