Measure the existing table top. Use a jigsaw to cut the new top from 1/2-inch-thick, medium density fiberboard. The new top should be no more than 30 inches longer or wider than the existing table top.
Cover the existing tabletop with a tablecloth to protect its surface. Center the new top on the table. Get under the table and trace the outline of the current tabletop onto the new tabletop.
Cut four 1-by-2 inch pieces from the scrap fiberboard. These will be the "stops" that keep the new table top from sliding around.
Lay the fiberboard tabletop on a flat surface with the right side down. Place one of the stops in the center of each side of the outline, with the inside long edge running along the outside of the shape. The entire stop should be outside the outline.
Install a small drill bit in a drill. Adjust the length of the bit to 5/8-inch. Drill two pilot holes in the center of each stop, 1/2-inch from each end. Carefully hold the stop in place as you drill the hole.
Drive a wood screw into each pilot hole. Turn the table top over and place it on the cloth-covered table.