Don t remove the cap from a hot radiator. Wait at least 20 minutes for the engine to cool down and always use a rag to take off the cap. Check the Battery Wear eye protection. Unscrew the six small caps or pry off the bars with a screwdriver. The fluid should cover the plates.
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.