Select a quality black latex paint. Choose a flat finish, which reflects less light. It should also be durable. Most important of all, it should be inexpensive, since you will be coating and re-coating the floor all the time, either to replace the black with another color or to replace another color with black. The default paint should be black because it is the color most likely to be desired for a play.
Remove any props, curtains or other objects that might get in the way as you paint.
Apply the paint using a roller brush. Start at one side of the stage and work your way to the other side, making sure you don't paint yourself into a corner. Let the paint dry.