A flat panel solar water heater is the most common homemade variety because it's simple and inexpensive to create. Most flat panel collectors are made with a wooden box frame, a metal or plywood backing, insulation, small metal pipes against the backing and a cover of glass. The size of your collector frame determines how much water you can heat at once, and how hot you can get it. The larger the frame is--and the longer it takes the water to travel through the collector--the hotter your water can become. The plywood or sheet metal makes the bottom or back of the box frame. You need to paint this black so that it can absorb more of the heat from the sun. Put together standard 2-inch-by-4-inch or 2-inch-by-6-inch boards to make the sides of your box. Mount coils of metal pipes--copper conducts heat very well--inside the frame against the backing. You can coil these metal pipes in a circular pattern or weave them up and down like a small radiator. Paint the pipes black as well so they'll collect the sun's heat more effectively, then cover the entire box with glass. Apply insulating board to the back of the plywood or sheet metal backing to help hold in the heat that's generated. Set the entire assembly in a sunny south-facing area, and pipe water into one side of the internal coiled pipes, then out the other side. The sun enters through the glass facing and heats up the pipes and backing, then as water slowly travels through the metal piping it collects the heat that's being generated.
Using metal for your box frame backing and water piping will create the most efficient water heating system because metal collects and distributes heat very easily. Using plywood for the collector backing works too, however, and is a popular variation for both price and material availability. Nonmetal materials are often used for the water piping as well. You can use a garden hose for the internal coil. Various types of plastic are also popular to use. If your hose or plastic piping is not black already, you'll get the best results from your water heater by painting them before assembly.
Homemade solar water heaters can generate enough heat to scald and burn you. If you find your water heater routinely generates extremely hot water, you may want to add a regulator valve to the outlet to prevent serious injuries.