I use Claude's fajita marinade. It is very spicy, so if you don't like yours hot then you need to add a little water to it.
I usually slice 3 large chicken breast thinly, and pour about 1/2 cup of the marinade and add 1/4 cup of water then stir it up and let set a couple of hours.
Put in hot cast iron skillet and cook until done. Remove from skillet. Then to the same skillet, I add a tablespoon of oil and saute my onions and bell peppers.