Preheat your smoker to 225-260F. While its heading up, begin to trim your brisket. Trim down the fat side, until you have about 1/4 inch of fat all through the fatty side. Turn to the lean side and trim off any silver skin and any hard chunks of fat. Trim off any piece of meat "stragglers" that will end up burning in the cook. The goal is to make it look as aerodynamic as possible.
In a seasoning shaker, combine kosher salt, pepper, granulated garlic, celery seed, paprika, and cayenne pepper. Apply a thin coat of mustard on both sides of your brisket, then shake on a liberal coat of rub to both sides of your brisket.
Place the brisket in your smoker and place a probe in the point and flat sides. I like to spritz it down with a 50/50 water and apple cider vinegar blend about once an hour. Smoke until you get a dark bark, then pull off and wrap.
Return to the smoker, and continue your cook. Probe for doneness once an hour until it feels like room temperature butter. You'll see a internal temperature in the 200-210 range. For a 15 lb brisket, I had about a 12 hour cook and internal temp of 204 when I pulled.
Let brisket rest for for an hour or two until internal temperature is around 150-160F. Slice into 1/4 inch strips for the flat, 1/2 strips for the point, and serve. Enjoy!