The same way any charger does, except that all you have to do is plug them into a 110V AC elec outlet. Once a specific level of charge has been reached, the indicator lights will show you whether the battery is charging or totally charged. Once totally charged, the charger will "monitor" the charge level & maintain it at that level.
NOTE: you shouldn't charge a battery when the water level is below the tops of the lead plates, until you add enough distilled water to cover the plates. And you should wait until the battery is charged before adding distilled water to "top them off" ... and don't add distilled water until it reaches the bottom of the hole over each cell, either, because a battery being charged will cause the liquid to expand and it may overflow out from under the caps. Always use distilled water, as tap water may have minerals, chemicals or particulates in it that will eventually reduce the charge capacity of the battery.
... cp