Measure your swimming pool if you do not already know its dimensions. Solar covers are manufactured and cut in common swimming pool sizes.
Visit your local pool supply retailer and purchase a solar cover. It will look like rolls of light blue bubble wrap. The cost varies depending on size.
Place the cover bubble side down on the pool surface. Remove the cover when you are swimming and adding chemicals to the pool. Otherwise it can stay on the surface of the pool, eliminating evaporation and warming the pool water as well.