Thicken the soup by adding a slurry - using either flour or cornstarch. A slurry is 1/2 cup liquid (usually water) with about 2 tablespoons of flour or starch. Mix these together in a measuring cup and add it to the potato soup. Flour thickens at a lower heat than cornstarch, so if you use a flour slurry, add it on low heat; if you add a cornstarch slurry, turn the heat up when you add it.