To make sour milk, stir about 1 teaspoon of white vinegar or lemon juice into 1 cup of milk. Let the mixture sit for a few minutes. When the milk has visibly curdled, it is ready to be used.
Some recipes call specifically for soured milk, but sour milk is also a good substitute for buttermilk. Adding vinegar or lemon juice to milk affects the taste and increases the acidity, bringing the flavor and chemical composition close to buttermilk. After souring milk, use it right away, and discard any leftover milk. Soured milk is not intended to be a beverage.