Remove the offending cushion from the sofa. Vacuum the sofa under the cushion to remove sand, dust and crumbs.
Measure the width and length of the cushion, then subtract 6 inches from each measurement. Write down the measurements. For example, if your cushion is 24 inches long and 18 inches wide, the measurements are 18 inches long and 12 inches wide.
Cut the rug mat or shelf liner to the new dimensions. By cutting the material smaller than the actual cushion, you eliminate any chance the material shows under the edges of the cushion.
Center the rug mat or shelf line on the sofa where the sofa will be.
Place the cushion on top of the material. Do not to slide it into place; sliding it in rolls the no-slip material. To prevent this, lower the cushion into place.