For every cup of buttermilk you want to make, stir 1 tablespoon of lemon juice or vinegar into the milk.
Let it sit for 10 minutes then gently stir and use.
Notes
Other Buttermilk Substitutes
You can also use lactose-free milk in place of regular milk when making a buttermilk substitute at home. It might be a little sweeter than one made with regular milk, so keep that in mind.You can also use sour cream as the base. Mix 3/4 cup sour cream with 1/4 cup milk. Stir well. This will have a similar tanginess to buttermilk.Kefir is another good base. Again, mix it with acid in the same proportions: 1 cup kefir with 1 tablespoon vinegar or lemon juice.