In the sample piece, I have chosen to backstitch 6 threads or 3 stitches from the finished stitching. Your stitched piece will largely determine the size of course. If you want to use a stitched piece for the back also, backstitch this in the same way being careful that both pieces are the same size when backstitched. Fabric backing will not need stitched.
Iron the piece and trim to ¾-inch from the backstitching. Cut fabric backing to the same size.