As said above,a flat is maybe the most difficult to cook consistently well.
The CS was developed as a brisket cooker.
I recommend that you keep it as simple as possible,when you first cook anything.
Take good notes and make simple changes with each successive cook.
Do a search at the top of the page for brisket,or brisket flats,and it should help a lot.
Once you get the cooking technique down,than there are a lot of changes you can try.
This being said,I've cooked some passable choice large flats in a CS with a a coating of worcestershire,a simple rub of salt,pepper,some granulated garlic and a little brown sugar.
I stay away from mesquite on long cooks and prefer 3 or 4 oz of nutwoods.
Here is a good starter for a brisket flat,for your first cook.
Brisket Flats 225� is always a good place to start cooking many things.
Hope this helps a little.