Different ways to do it.
Vinegar and salt is one Soak in vinegar add some salt small amount maybe an hour or so then insert into an oven that is preheated and leave 20-30 min then pull out and re-soak in vinegar for a couple of min.
Another way is to get a closable container put a rag or paper towels in the bottom and soak the rag with ammonia. Then put the brass in there and cover it, don't remember how long to keep it that way.
Another way is with acid type solutions. You can buy on Amazon they call it Brass aging or ager solution or something like that.