1. If the recipe calls for 1 cup of buttermilk you need one cup of milk and 1 tsp of vinegar or lemon juice.
2. Add 1 tsp vinegar in milk at room temperature.
3. Give it a gentle stir and let it sit for 5-7 minutes. You will see the milk has curdled.
You can use buttermilk in baking breads and cakes to make them soft and fluffy.