If you're using pre-made coasters to make these, those coasters will already have feet at the bottom of them. But, if you're using something like ceramic tile, they won't have feet at the bottom and could potentially ruin your tables. So, you have two options. Option 1: Cut some felt into four small pieces and affix the felt in each corner. Let the glue dry overnight. Option 2: Purchase self-adhesive felt bumper dots. These are anti-skid dots that you usually affix to things like kitchen chairs or flower vases. The pads help to assure that these items won't scratch surfaces, which is exactly what you're trying to prevent by adding feet to these coasters.