Add milk to the measuring cup until reaching the 1 cup level.
1 cup milk
Stir milk and let sit at room temperature for at least 10 minutes.
Notes
Whole milk will yield the best results in consistency for sour milk and will provide the most consistent buttermilk substitution, but low-fat milk or non-dairy milk can also be used.
Freshly squeezed or bottled lemon juice can be used.
To make more than 1 cup of buttermilk substitute, use 1 tablespoon of lemon juice for each cup of milk you need and add enough milk to measure the final measurement needed. For 2 cups of sour milk, use 2 tablespoons of lemon juice and enough milk to reach the 2-cup measurement line.