To make buttermilk, follow these three recipes.
Recipe 1:
Take some yogurt and blend it when it smoothens, add water and blend it again, and churn until there is a separation of butter. Separate the butter, and the leftover is the buttermilk.
But if you use milk and add up some lemon or vinegar, it causes the separation of milk into cheese and water. So not a way to make the butter milk.
Recipe 2:
Some people use milky by-products to make buttermilk.
Churning and converting cream into butter by blending it, milk is fermented with some bacteria.
Recipe 3:
Skimmed milk is also a good substitute for buttermilk.
By putting fermentation into the milk and rest it overnight at room temperature. But choose right bacteria for it.