Generally, for each cup of medium-thick sauce, use two tablespoons flour mixed with 1/4 cup cold water. Or use one tablespoon cornstarch mixed with one tablespoon cold water. Be sure to thoroughly mix the water with the starch (the flour or cornstarch) to prevent lumps. After stirring the combined starch and water into the liquid to be thickened, cook and stir over medium heat until thickened and bubbly. Cook and stir one minute more for flour and two minutes more for cornstarch to completely cook the starch.