Most batteries have a fill indicator inside each cell that shows the correct level. Use distilled water to fill the cells. The holes to fill a battery are pretty small and it s easy to make a mess unless you have something that allows dispensing a small amount of water at a time. Best is a battery filler - inexpensive and available at auto parts stores, but a small tin can that has been squeezed to make a narrow opening works pretty well, too. Do not smoke or otherwise have an open flame near a battery.
Batteries generate hydrogen and can explode.
Do not get any of the battery fluid on you or your clothes. It s sulfuric acid and will burn your skin and your clothing, giving you a rather Les Miserables-like appearance. If you do get exposed to it, flush the area with a lot of water immediately. If you have a sealed battery, there is no need, or possibility, of adding fluid. Many of these batteries have charge indicators that give an estimate of their condition, however.