How do I lay the stone for a short stone wall around my flower garden. I have plum red flat stone 1-3" in thickness. I want to have the wall about 12 inches high and one stone's width (stones are about 12-15" wide) Do I back fill with topsoil first to bring up the flower beds or do I put up the wall first. Do I put down something under the stone i.e. a layer of plastic to stop weed growth, sand to set the stone. Any help would be appreciated. Thank you.