It's very easy. You should try it.
All you do to start off with is to buy some buttermilk from the store.
Here's the recipe:
How To Culture Buttermilk
1 quart whole goat milk
2 tbsp. cultured buttermilk
Warm the milk to 72 degrees. Stir in the cultured buttermilk. Allow
mixture to set, covered, at room temperature for 12 hours or more. When
thick, refrigerate.
Use store bought cultured buttermilk or a buttermilk culture from a
cheesemaking supply company to start your first batch. When purchasing from
a grocery store, be sure to get the freshest you can. Check the expiration
date. Be sure to save some from the previous batch to reculture the next one.
Just like yogurt, buttermilk can be kept going for a long time.