Best way is with a filling system where you just pump the water in and the caps stop the flow when full ;)
Otherwise just open the caps and check the fluid level. Most important part is eye protection. Fill to 1/4" to 1/8" below the split ring that extends down into the cell. Just touching would be max and may still push some electrolyte out during charging. Best to check water level when battery is close to full charge.
Distilled water only. The minerals for taste in bottled water will degrade the battery. Especially any minute amount of iron.