If you don't have an electric knife you can use a straight edge and a box cutter/razor knife. Just score it several times until you are through the foam. Be sure to do this where the razor won't harm what is underneath. The best thing to attach the two pieces of foam together is spray contact adhesive. Follow the directions on the can to spray, let it dry, then put the two together. As far as attaching the batting, you can spray glue it as well. Covering it could be as varied as the looks you could want. I personally like boxed cushions with welting and a zipper but anything goes.