2-ply rag in strips as wide as you need spacing. Line the inside of the frame with Saran wrap and start placing the strip around the inside. You can pin the one end. when you start the overlap, apply rice or wheat paste. I usually stop on the second layer, and let it dry over night. The next day I can whip out the other 4 layers resulting in a spacer slightly fatter than 12-ply mat. (about 5/32") and perfectly matched to your frame. Before you remove, make a mark on the spacer and frame to match them up later. Now color.