I just cut both fabrics approximately 7" wider and longer on each side of the table. I sewed the corners like the fitted sheets (or the fabric boxes everyone was making). Serged the edges then zigzagged the elastic, I used 1/4", on the ends. Make this as tight as you can stand it and it will hold the tops in place. You could also run a casing all around the bottom, run the elastic through, then pull as tight as you want and tie it.