The instructions say to add some plain whole-milk yogurt to heavy cream and allow it to sit a day.
The second day you just use a mixer, whip and "presto" butter forms! The butter has now separated from the "buttermilk." The buttermilk is poured off. The butter is rinsed several times in ice water. With each rinse you work the butter with your hands or a paddle under the ice water to try to get out all the buttermilk.
Then you can add salt and any herbs or flavorings you want.
The only thing I added to my butter was salt, and I poured the buttermilk into the jar the cream came in.
I'll use the buttermilk for pancakes or some other recipe and we're totally enjoying the butter already. I just added salt to it. This was fun to do, although a bit expensive for two pints of the wonderful cream.
See the site for more details if you want to try this yourself.