Whisk everything except the flour or cornstarch together in a mixing bowl, set aside.
Prepare your stir fry (meats and vegetables). Be sure that the meats are cooked through and the vegetables are however you like them, usually crisp-tender. Pour the prepared sauce over the vegetables and heat for 1-2 minutes.
If the sauce is too thin, make a slurry of 1 tablespoon flour or cornstarch to 2 tablespoons water. Mix it together with a fork then stir the goo into your sauce and heat for another couple of minutes. The sauce will thicken up and be just fine.