Reduce the sauce that you plan to thicken.
In a separate bowl, stir water into the flour, a little at a time, until you have a smooth mixture.
Pour the mixture into the sauce.
Boil the sauce for a couple of minutes until it thickens. Then, strain the sauce.
Check that the consistency of the sauce is uniform and velvety. This sauce may be kept in the fridge for 2 to 3 days.
Starch, as compared to flour, is flavorful and makes for shinier sauces.