I fold each piece of the "sandwich" in half the long way and mark the centers with a pin, then fold them in half the other direction and mark centers with a pin. As I smooth out each layer I center them on the pins. This is the process I use whether the backing is pieced or not. This way I know I won't end up with one side being too short or something. If I were to do sandwiching on the floor, I would stabilize each layer with painter's tape as I smoothed it out. I do have a table that works for me, but I still use the tape on the backing until I get it pinned. Good luck with your pieced backing, it should work for you!