Choose a sauce base: stock, wine, cream, tomato, or pan drippings
Pour the liquid into a wide saucepan or skillet
Bring it to a simmer over medium to medium-high heat
Stir occasionally to prevent sticking or scorching
Let the liquid evaporate until it thickens and reduces in volume
Lower the heat if it starts boiling too hard
Skim off any foam or impurities if needed
Taste the sauce as it reduces
Add salt, pepper, herbs, butter, or other seasonings near the end
Stop reducing when the sauce reaches the desired consistency
Remove from heat and serve immediately or strain if desired