Brisket its all about 4 things:
- Being able to hold a steady temp for 13-16 hrs. (on average).
Start with a full packer (flats are smaller and leaner so they are less forgiving with drying out) in the 12-15 lb. range. You can either trim the fat or not up to you - if you do trim the fat I highly suggest placing it on the rack above your brisket so it can drip down on it and baste it. Put on a simple rub salt, pepper, granulated garlic, paprika, and a little cayenne powder if you want extra kick. Go big on your rub flavors - it's a big piece of meat and your seasoning the outside only.
Run your smoker at a steady 225-235° put the brisket on, close the lid..... AND LEAVE IT ALONE!! Don't mop, spritz, peak, poke,... you get the idea. Just open the smoker to add wood, and stick the probe in around the 4 or 5 hr. mark. Once you hit 190° internal temp stick the brisket with a skewer or toothpick, if it slides in nice and easy your done, if not keep going (check 3 or 4 places along the brisket). Once it is done wrap it in foil and let it rest for at least 1 hr.
The main mistake people make is they don't leave enough time to cook and then with all the family and friends arriving they panic and rush it. That gives you tasty shoe leather. Better to finish early and hold it in a dry towel lined cooler for a few hours than rush it.