If you do a search on "trimming brisket" you will find several posts discussing the options. Some trim heavily, some lightly. I can tell you what I have done and gotten good results with - but it is not the only way to trim a brisket.
1. I start with the top side (non fat cap side). I remove the outside fat that is hard, tough and often slightly yellow - as this will not render and just acts as a block to getting smoke in. I will go down to the muscle so that there is little of this left - looks mostly red meat.
2. I carve into the fat layer separating the two muscles as much as I can without separating the two pieces. There is a lot of fat here and I can afford to get rid of a lot of it.
3. I look for any silver skin and remove all that I find.
4. Then, I move to the bottom side (fat cap) and trim away any hard, tough fat, and any fat that is tied together with silver skin or membranes )(it looks wrinkled and you can grab it and pull it around and it does not cut well). Good fat for rendering and lubricating your brisket slices cleanly and is easier to cut. I like to leave around 1/4 inch of fat cap when I am done.
Basic trimming like this will reduce the length by between 1/4 - 1/2 inch or so. If that is within an inch or so of being the right size, I might let it go, as it will shrink some. If you are 2-3 inches off, it won't shrink that much usually.
I have never cut down a brisket - so if someone who has done it has a better idea - listen to them! If I was doing that, I would take it from the big end of the point (the larger muscle). I would never waste any of the flat muscle, and, for me, the big end of the point gives you better slices, but not as juicy usually.
Another option is to split the flat and point into separate muscles and see if they fit on two different shelves.
Lastly, I always keep my briskets out of any pans - so they get air (smoke) circulation all around.