When it's done cooking, the grease will have melted into the roasting pan. Assuming you cooked it in a sauce, the fat will floating on top of the sauce.
Remove the meat from the pan and set it aside, covered loosely in foil to rest for 10-15 minutes. If the sauce is a thick and chunky one, tilt the pan so it accumulates in a corner and let it rest for a couple of minutes so all the sauce collects in one place and the fat all rises to the top. You can then skim it off the top with a large spoon. When you get most of the fat off, you can get the last of it by soaking it up into a paper towel which you can discard.
If the sauce is thinner, it might be easier to pour it off into a fat separator cup that enables you to pour the sauce out from under the fat.
The separator looks like a measuring cup with the spout connected to the cup at the very bottom. As the fat rises to the top, you can pour the fat free sauce out from under the fat.
Be sure you slice the brisket across the grain to ensure it is tender.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan