I don't know if there is a "proper" way to trim a brisket for BBQ. If I decide to trim one I try to make the fat cap even across the entire surface, taking off only areas of very thick hard white stuff. On a choice cut you might not even want to trim sometimes. Triming really only eliminates excess fat in your finished product.
As pappy said you just want to get rid of the excess fat. Trim to approx 1/4" thick. Get rid of any 'hard' fat. I scored the fat on mine in a cross hatch patter but that was just because I wanted to try and get some rub closer to the meat.
Some people 'mark theirs to know the direction of the grain. I don't bother with that.
I've got to agree with Chris. Trim the fat cap down to 1/4". The last brisket I did had slightly more than that on it and the fat did not render out which meant I had to cut the fat off along with all that great carmelized crust from the rub.
The proper way to trim a brisket, is to trim it the way you like it! But I do as Chris suggested, except for the scoring process. Sounds like a good idea though, even if Finney thought about it! 8-[
Sometimes I'll trim some fat off the bottom and then put it on top of the brisket for a nice "blanky" effect...doesn't hurt, but not trimming ain't gonna hurt either! :!: