I'd season it now for cooking tomorrow (you'd want to allow a good 6-7hrs to cook). If you've got some fancy Texas spice rub, go nuts. More often than not, I just use liberal amounts of salt & pepper. Tomorrow about 8hrs before you want to serve, I'd preheat the oven to 110⁰C. In a roasting pan I'd lay a couple of sliced onions before adding the brisket. The onion just stops it touching the bottom of the pan & burning/dying out. It'll sweat for the first few hours but it should be starting to brown somewhere between the 3rd & 4th hour. By the 5th hour it should start to look appetising & ready but it's not, it'll still be really tough. The safest bet is to cook it for a long time & mitigate the chances of it drying out (a particular problem with the brisket flat). At hour 5 you can think about covering it with the lid or foil. This keeps the moisture in but will also stop the bark/brown outside developing further. Adding half a cup of water now will also keep it moist. I'd want to go at least 7hrs straight but every oven is different.