Measure the size of your pool with a tape measure to determine what size pool cover you need. The cover needs to fit over the surface of the pool precisely, and in most cases must be cut to specification after you buy it. At this stage, you simply want accurate measurements for comparison purposes. If you have an abnormally shaped pool, don't panic. Simply measure it at its widest point and at its longest point; that's sufficient to determine the right size for your pool cover.
Contact a pool supply store and see if you can purchase a solar pool cover that matches your specifications exactly. If you can't (and you often won't), then purchase one larger than what you require. You can then cut it to match your pool's size.
Lay the pool cover flat across your pool--bubble-side down--taking care to smooth out any wrinkles or folds. Mark the location of the pool sides along the cover with a magic marker.
Cut your pool cover to size with a pair of scissors, using the markings you have made as a guide. You want the cover to match your pool specifications exactly. A good rule of thumb is to make two series of cuts: the first one a "general" sizing, and the second a "trim" to make sure it all fits.
Lay the re-cut pool cover across your pool. It won't sink, and the seal it makes with the water will help it to work its best.
Stand on one end of the pool and pull a length of the cover measuring about 1 foot up onto the patio. Make a fold and pull another foot up, then fold it again and pull another foot up, and so on until the entire cover is up. This "accordion" folding takes up a minimum of space, and will allow you to quickly replace the cover across the pool when you're done swimming.