I also pin on the carpet - I know, right??? Seems like you can't do it that way - but I pin the corners and outer edges of the backing, right side down then layer the batting. Then, with the top folded in half, right sides together, I lay it on top of the batting, matching the center of the back. Now, here's why it works - I slide my cutting mat UNDER the backing so I have a hard surface to pin to when pinning all three layers. If I have to take out a pin attaching the back to the carpet in order to fit the cutting mat under, I just pull the backing tight again and re-pin. Move the mat around under the sandwish and pin like crazy!! Oh - and pin from the middle outward.