It's so easy! Buttermilk is so expensive! And we go through SO much buttermilk in a month I don't know why it has taken me all my life to learn this...
To make buttermilk.
1. Buy buttermilk. For the last time though! You have to have something to start with...
2. Get cleaned quart jar.
3. Fill quart jar to second notch, or one cup of buttermilk in quart jar.
4. Fill remainder of jar with milk, preferably whole, but I use what is in my fridge, which is 1 or 2%.
5. Put a lid on it.
6. Shake the jar a bit to combine buttermilk and milk.
7. Set on counter for 24 hours.
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BUTTERMILK!
A friend told me how to do this process, but I just did a quick search and found this link which provides a bit more detail.