Cornstarch thickens dishes that are in appropriate for roux, or equal parts fat and flour, such as puddings and glazes. It also can be used to replace roux in others, such as soups and sauces. Cornstarch doesn't impart a flavor or aroma when it thickens, but it doesn't work alone; you have to mix it with cold water to form a slurry. You need a 1-to-1 ratio of cornstarch to water to thicken a sauce.
Mixing the Slurry
Whisk 1 tablespoon of cornstarch with 1 tablespoon of cold water for every cup of sauce you want to thicken a few minutes before you finish cooking it. Store the slurry in the refrigerator while you finish cooking the sauce.
Thickening the Sauce
Finish cooking the sauce and season it to taste. Lower the temperature of the stove until the sauce just stops simmering. Take the slurry out of the fridge and whisk it again to suspend the starch granules. Pour it into the sauce a little at a time, whisking constantly. After you add the slurry, cook the sauce for about 4 or 5 minutes so that the starch granules completely gelatinize.