Stand a cup of milk in a glass jar and squeeze in the lemon juice or add the vinegar and gently stir this mixture. Keep it at room temperature for 10 minutes without stirring or shaking it.
In 10 minutes, you can check that your milk has coagulated and formed a slightly curd-like thick texture.
Stir and the result is a beautiful buttermilk that can be stored in the refrigerator for 3 days.
For making Buttermilk from Yogurt
This one is really simple! All you need to do is churn the yogurt and whisk in a little water to get a creamy textured buttermilk.
Notes
You can use any kind of milk that you prefer for this recipe. Skim, 2%, almond milk, whole milk all work great.
Buttermilk can be used to make a variety of things and it adds flavor to not just baked products but also enriches any fried, steamed and pan seared recipe. It is generally used in pancakes, cupcakes, to marinate chicken, as a brine for fried chicken, moist sponges, idlis, rice dishes and many more.
I recommend using this within 3 to 4days of making it, just to be on the safe side. Always keep refrigerated.