The key ingredients in El Pollo Loco marinade for chicken are chilies, garlic, oregano and lime. Yellow food coloring can be added so the chicken not also tastes like the Pollo Loco chicken but looks like it too. You can omit this ingredient if you prefer.
Take the stem out of the chili and remove the seeds too. You can then mince the flesh finely. I recommend wearing gloves for this. If you don’t have any, wash your hands straight after handling the chili. You don’t want to rub your eye, for example (and yes, I know this from experience!)