Is the backing pieced?
I would want to keep the seam straight on the backing so would start around the seam line. Spray basting will help keep the sandwich together. You will still need to keep smoothing the layers from wrinkles.
So, I would start in the mid center. Baste out a square area to work on. This will help keep the layers together. SITD and quilt as desired, remove the basting box, then proceed to the next section.
Another easier option is to divide in four quadrants and work on each section.
On a resent project, I basted a gold thread down the center of the backing to keep the design straight. Easy to remove when finished. Good luck!