If you are referring to smoking flats, that is mainly what I cook because it is mainly the only part of the brisket I can purchase in my area. The size of a flat can range from 4lbs to around 7lbs.
I generally will season with kosher salt, rough ground pepper, and some granulated garlic. I trim the fat down to about 1/8" - 1/4", season the meat about an hour before placing it in the smoker. Preheat the smoker to about 250°F, place the flat in the smoker, and cook at 225°F - 250°F, until an internal meat temperature of 160°F - 165°F. Remove from smoker and tightly foil with heavy duty aluminum foil. Before sealing add some beef broth or water. I then check when it gets to 190°F; by using the fork test. If a fork is easily inserted and removed, then it is done. If not I will continue to cook, and check every 5 degrees, but you generally don't have to go above 205°F. Unfoil and allow to rest at least 30 minutes, and slice thinly across the grain of the meat.
The time when it will be done can be unpredictable. Most will cook a day ahead, or will start several hour earlier than they expect it to be done. Look up FTC; which is one method of holding your brisket at a safe temperature until it can be served.