Source heavy whipping cream (any amount - but I prefer to get one big carton to make a mid-size butter portion).
You will absolutely need a mixer (hand mixer/stand mixer). You can try the shake jar method but holy smokes it takes forever.
Dump all your whipping cream into the mixer and simply let it mix on medium speed until the fat separates. I advise wrapping the bowl with some saran wrap or a shield because the water splashes everywhere.
Once the butter is separated, take it out with clean hands and run it under ice cold water while mushing it. This helps get any leftover liquid out of the air pockets.
Transfer to any container of choice (keep in mind this butter will go sour if you leave it out for more than 3-4 hours at a time). I love using the butter dome thingy.
Sprinkle with your choice of salt (I highly recommend Maldon Salt or Truffle Salt). It makes this butter divine.