How to Make Buttermilk at Home: You can make 1 ¼ cups of homemade buttermilk by using regular milk + either lemon juice or white vinegar. To do so, pour 1 tablespoon of freshly squeezed lemon juice or 1 tablespoon of white vinegar into a measuring cup. Add enough milk to equal 1 ¼ cups. Give it a stir, let it sit for about 5 minutes, and then use it in the recipe as directed.