Step 1: Trim the brisket, leaving a 1/4-inch thick cap of fat on top. Place the brisket on a sturdy baking sheet.
Step 2: Mix the pickle juice and mustard, if using, to make a thin paste. Squirt or brush this mixture on the brisket on all sides, slathering it over the meat. Generously season the brisket on all sides with salt, pepper, and hot red pepper flakes.
Step 3: Set up your smoker following the manufacturer’s instructions and preheat it to 225 degrees. Add the wood as specified by the manufacturer.
Step 4: Place the brisket fat side up in your smoker. Cook until the outside is darkly browned and the internal temperature is about 160 degrees, about 10 hours, replenishing the charcoal and wood as needed.
Step 5: Wrap the brisket in butcher paper (nonplasticized, please). Return it to the smoker. Continue cooking for 2 to 4 hours depending on the size of the brisket. You’ll need to unwrap it to check it. When cooked, the brisket will be tender enough to pull apart with your fingers, with an internal temperature of about 195 degrees.
Step 6: Transfer the brisket to an insulated cooler. Let rest 1 to 2 hours. Unwrap the brisket and transfer to a cutting board, reserving any juices that accumulate in the butcher paper.
Step 7: To serve, trim off any burnt edges and dice. (You’ll serve these as snacks to warm people up for the meat.) Trim off and discard any large lumps of fat. Holding your knife parallel to the cutting board, cut the deckle (top section) off the brisket flat. Scrape away and discard any fat or collagen between them. Lay the deckle atop the flat, rotating it so that the grain of the meat lines up with that of the flat. Cut crosswise into ¼-inch slices and serve with any brisket juices drizzled on top.