You mean you want to make Carne Asada out of some Flank or Skirt steak? IE you want to make some fajitas? Yes?
Seasoning and marinating is up to you but skirt and flank are easy to cook. You can search far and wide for marinades from just lime juice and salt to more complex.
Easy way to cook it is hot and fast. Make note of the meat texture and which way the meat is running, because once cooked you need to cut ACROSS the run not with it, otherwise you are in for a chew fest.
I normally salt mine after it is sliced thin, easy to buy that way in South Texas though.
Throw is on a super hot grill for like 75 seconds a side and its done. Seriously don't overcook it. Just char the outside. The idea is it should be pink and juicy when it first comes off. Let rest then cut across the grain.
Add tortillas, guac, salsa etc. I am old school and just do fresh white onion and chopped cilantro. Sometimes a simple chimi churi sauce.