Determine how level the floor is by placing a level in the middle and on the sides of the floor. Determine which areas of the floor as not level and mark them accordingly.
Place a drop cloth on the floor.
Put on work gloves and protective eye goggles.
Use a mixing drill to mix the plaster mix and water in the bucket according to instructions listed on the plaster package. You need to work fairly quickly to be able to spread the plaster with a wet trowel.
Pour the plaster onto the floor, and smooth it out with a wet trowel. Place more plaster on the areas where you noted the floor is not level. You must estimate how must plaster is needed in a given area, because there is no precise way to do this. Allow the plaster to dry, and use the level to determine whether the floor is even. If it still is not level, mix more plaster, and smooth it over the parts of the floor that are not level.