I'd say start in the center and branch out, just because then you can tell yourself that the worse part is over and it can only get better. While you work you will promise yourself several times that you will never do this again, but I am sure you will. Make sure to take breaks and if by any chance you can not finish, give them a picture and an IOU card. Don't stress yourself out and make sure you check the back often to make sure is not bunching, because even if you use plenty of spray, it can bunch some. Also keep you eye out for the edges. You don't want the backing to fold back and quilted into the back of the quilt...Don't ask me how I know. Make sure you are listening to the machine. It may sound different when you add an extra layer to the quilting accidentally.
Good luck and show us pictures.