Put Brisket in large oven tray.
I put it in my oven at 200 C electric uncovered for 30 minutes and then cover with silver foil very well (completely) turn it down to 110 C electric and leave overnight or in the Roasting Oven of Aga for 20 mins followed by Simmer Oven overnight.
I will get it carefully out in the morning (watch out for the liquid tipping out of the tray) and pour off the liquid. You could put the meat back in to keep warm, if you need the oven for Roasties etc as I do, then put to one side covered with foil and a couple of tea towels. Make sure you get out all the meaty bits from the tray.
Wait for the juices to cool a little and take off half the fat, I save it to use for the roast potatoes.
Place the juices in a saucepan and bring to the boil. Add the cornflour to a little water in a cup and stir to mix well. I use a couple of big heaped teaspoons and add enough water to make the mixture liquid enough. When the juices are boiling stir the cornflour briskly in and repeat if it is still too thin for a gravy. Season to taste. I have added a stock cube and water to the juices if the meat has not produced enough liquid but I usually don't need to. If the cornflour forms lumps in the gravy just pass it through a sieve.
Depending on how long the meat is out of the oven I will put it back in at about 170 C covered with silver foil to warm back up. Another thing I have done is to carve the meat and then put in a roasting tray and cover with the gravy. You can then reheat, ideally covered with a wet and scrunched up before flattening back out, piece of baking paper followed by a top layer of silver foil. A bit of a faff but it keeps the meat so fresh if that's the right word!
When ready simply carve and Apply to Sunday Face with all the trimmings until it hurts.