Yep, trim as much or as little as you feel is necessary. Some trim most of the fat away as it takes longer to cook with more fat. My opinion on brisket is the longer it takes to cook the better. In time you'll find what you like best. I have trimmed the point muscle more so it's not as thick, then leave more fat-cap on the flat...this helps them cook a bit more evenly, as the point is much thicker than the flat. When smoking whole brisket (and most everything else with a fat-cap) I like to cross-hatch score the fat, for several reasons. It allows smoke to penetrate more easily where the fat is cut, allows more dry rub into those same cuts, and lastly, allows the fat-cap to remain in a more stable orientation on the meat as it begins to render-down and shrink, keeping more of the meat covered while it finishes cooking. Sure the meat shrinks as it cooks too, but at times it doesn't shrink quite as much as the fat, especially when cooked over low & slow. Without scoring the fat can to bunch-up on one side or another, exposing the meat on the edge of the fat when it shrinks, leaving bare meat without dry rub on it...mostly just cosmetics, but looks count to me, too. With scoring, the fat can shrink and just open up small gaps, while more smoke gets in through the cuts. And, it looks pretty cool coming out of the smoker, too.
Here's a 15lbr that just went under the knife...lightly trimmed and heavily scored down to the meat...if you slip and go down into the meat a little, don't sweat it...I do it almost all the time as it can be difficult to judge the fat thickness sometimes:
Flat muscle, ready for slicing...cross-hatch scoring on the fat-cap showing itself more when fully cooked:
As SmokeyMose mentioned, brisket is pretty forgiving. If there's a way to easily screw one up beyond recognition I think I'd of found out how that can be done by now. With some fat-cap on and cooking to reasonable finished temps, it shouldn't be dry or tough. 190* internal temp fro slicing is ball-park. Some go higher some go lower, but each piece of meat is a little different. Probe them for tenderness when you think it's ready, and if the probe slides in easy without resistance, then it's time to wrap in foil, towels and cooler it up for a few hours to rest and get even more happy. Resting is almost as important as finished probing for tenderness...resting keeps them tender and helps keep them moist when served.
Oh, I like to slice my meats immediately prior to serving...on demand, if you will...less time for exposure to air = less moisture evaporated from the meat. Keep covered when not slicing.
Enjoy your first brisket!!!
Eric