Measure the length and width of your pool. For odd-shaped or round pools, measure at the widest point.
Get a tarp that is 6 inches larger than the pool on all sides. Select a heavy duty tarp made for extended outdoor use. If your pool is larger than the tarps available, select two tarps that fit the measurement.
Cut a 12-by-12-inch square out of the center of the tarp. Cut a 16-by-16-inch square of nylon netting. If you are using two tarps, cut a 12-inch-wide strip of netting that is equal in length to the side of the tarps.
Lay the netting over the hole or between the sides where the two tarps will attach, as necessary. Cut a length of 2-inch wide nylon strapping for each side of the net square or strip. Pass each cut edge of the strapping through a candle flame so it melts slightly to prevent unraveling.
Sew the netting and strapping to the tarp, sandwiching the netting between the tarp and straps. The straps reinforce the edge of the net and the cut edge of the tarp. Use a heavy-duty needle and thread rated for outdoor use. The netting forms a window in the pool cover for water to drain through, otherwise the water collects in the center of the cover and causes it to sag.
Lay the cover over the pool. Attach one end of a rubber tie-down or bungee cord to each of the grommets around the side of the tarp. Attach the hook on the other end of the tie down to the tie down bolts surrounding your pool, pulling the tarp as taut as possible. For above-ground pools, attach the tie-downs to stakes in the ground around the outside of the pool.