I piece my backs with a seam directly in the center of the back so that when I place my top down I align the center of the top with the center of the back. Sometimes I put some of the big safety pins along the seam on the back that I need to find, it makes it easier to feel through the batting. It usually takes me a few tries before I get mine where I want it. You can always tell when I'm sandwiching because I'm usually not a happy camper, but once I get it where I want it I'm super glad I took the time.