To make buttermilk from regular milk, add 1 tablespoon of white vinegar or lemon juice to 1 cup of milk. Stir the mixture and let it sit for 5 to 10 minutes. This will cause the milk to slightly curdle and thicken, creating a buttermilk substitute ready for use in recipes. The acidity from the vinegar or lemon juice simulates the tangy flavor and texture of buttermilk, perfect for baking or cooking needs. Variations include using sour cream, cream of tartar, or even dairy-free milk alternatives with the acid to achieve buttermilk as well.