The top is completed, should be squared and the backing should measure 6" larger (this is my preference, I can get away with less on the sides but need the full 6" on top and bottom to allow room to attach to my leaders) on all sides and also squared up. No sandwiching or basting as that is done when putting it on the rack.
You can discuss with your LAer if you want to supply batting or if they will. Some LAers will also square up your backing for you but it is up to you to ensure your top is square. By square I mean your top end measurement matches your bottom end measurement and your right side measurement is the same as your left side measurement.