Here is the recipe we use. We think of it as a three day process although it could be speeded up.
Marinate brisket overnight in small bottle BBQ sauce, 12 oz beer, 1 package onion soup mix (rub on top, fat side up), 1-2 tablespoons of liquid smoke, and garlic if you like, salt and pepper to taste, allowing that onion soup is salty.
slow cooker all day on low, or oven 8 hours at 250. We like to then put in frig overnight and scrape off all the hardened fat, and it is easier to slice cold, then reheat in the good gravy left.