Add the milk to a jar then pour in a tablespoon of lemon juice of vinegar in and stir well.
You can set aside for 10 minutes then use.
If you are using this replacement for a recipe where the consistency is important like a buttermilk chicken or salad dressing then transfer to a sealable jar and shake well then set aside for 10 minutes, shake once more and use!
You can keep the buttermilk replacement in the fridge for a week or freeze it for up to two months. For a non dairy version substitute the milk for coconut milk.You can also use 3/4 cup sour cream or yogurt thinned out with 1/4 cup or water as a buttermilk replacement.If you don't have lemon juice or vinegar on hand you can mix 1 3/4 tablespoons cream of tartar into the cup of milk then follow the same steps above.