Stir the paint thoroughly with a paint stick. As you stir, scrape the top and bottom of the can to make sure the pigments evenly mix.
Add a paint conditioner to the thickened paint according to the manufacturer's instructions. Paint conditioner won't thin the paint; it makes it easier to apply.
Add 1 oz. of room temperature water to the paint can and stir the paint thoroughly.
Dip a small artist's paintbrush in the paint and paint a test swatch on a scrap piece of paper or cardboard. If the paint is still too thick, repeat steps 2 and 3, adding 1 oz. of water at a time until the paint is thin to the desired consistency.