Test your paint to be sure it needs to be thinned. Stir the paint well using the paint stir stick or other stirring tool. Then slowly pull the stir stick straight up out of the paint and allow the paint to dribble off the stick. If the paint flows off the stick evenly and without hesitation, you don't need to thin it.
Add water in small amounts if you determine the paint needs to be thinned. Begin by adding about two tablespoons per pint of paint.
Stir the water into the paint well and repeat the paint test. Add additional water until the paint flows smoothly off the stir stick.