Pour your heavy cream into a plastic container and whisk in the buttermilk. Cover and set on a flat surface for 24 hours at room temperature.
After 24 hours, check to see if the culture has fully incorporated (whisk again if there are thick and thin places). If not incorporated, leave out another few hours. You will know your cultured cream is ready to church when it is thicker and feels more like a yogurt consistency.
Once your heavy cream is thicker and fully cultured, place the plastic container into the fridge to reduce the temperature to 60º (can take 2 hours, or overnight like I do in the video). Once your cultured heavy cream is 60º, begin churning your butter.
You can whip your cultured cream on medium high to begin the separation process, but once the buttermilk is separating from your butter, you'll want to reduce the speed to avoid sloshing. I use the whisk attachment of my stand mixer.
Once the butter has begun coming together, exchange the whisk for the paddle, and slowly continue churning. Use a large towel to cover the stand mixer if the sloshing gets too bad.
Strain off the buttermilk (sweet whey) from the butter and return the bowl for more churning. You will be done churning when no more whey is produced. You can keep this whey to substitute as buttermilk in your baking but this won't last very long and must be used within a couple days.
You will now need to 'wash' your butter by pouring half a cup of cold water (to avoid heating up your butter) and allowing your stand mixer to push the water through your butter. You will strain this water off and repeat with more cold water until the water strained is clear. If you leave whey in your butter, it will sour and rot faster.
Once the washing water is clear, strain it off and salt your butter if so desired. Work the salt throughout the butter. Once the salt is fully mixed in, using a scraper, press the butter up along the sides of the churning bowl to allow the salt to push out the last of the water.
Allow the butter to sit like this for a while (I usually allow it for an hour at least). Your butter is now ready to either press into butter stick molds or into your butter bell to enjoy.
Notes
when working with live cultures, I avoid using metal as much as possible, it can cause a culture to become sluggish
you can churn your butter using anything you wish, as long as it will encourage the butter and buttermilk to separate and have a slow speed option
the buttermilk that separates from your butter is not true buttermilk, but a sweet whey, which can be used in a whole host of ways