how do i make a solar battery?

All that is, is a photocell which charges a rechargeable battery.

You need to find a solar cell which will put out enough voltage to charge the particular battery you wish to use. Either you can connect a number of solar cells in series to up the voltage or a number in parallel to up the wattage (provided your voltage is sufficient).

Probably easiest to charge a NiCD 1.5v AA battery. That will be easier than trying to charge a 12v car battery, for instance (and much quicker). Maybe you could charge two of them then show how they can power a small flashlight when done (or charge AAAs and power an MP3 player – lol)