Once you have a definitive cream line*, use a ladle or large gravy spoon to skim the cream off the milk. As you skim, add the cream to a large 1/2 gallon jar or mixing bowl.
Add the cream to your preferred small appliance, whether it's a stand mixer, blender, or butter churn. If you're using a mixer, you'll want to cover the mixer with a flour sack towel, or use a shield to keep it from splattering all over the place.
Allow it to run 'til the butter separates from the milk (less than 5 minutes for a blender, about 20 minutes for a mixer, and maybe 30 minutes for a butter churn); then drain the milk off the butter curds. You'll know when it's ready because you'll have chunks of butter separate from the liquid.
If the butter is too soft, add ice water to the churn and let the butter firm up before removing the liquid.
Remove the butter from the jar and squeeze out as much milk as possible.
Place the butter on a cutting board, and roll it with a rolling pin to squeeze out more milk.
Alternate cooling the butter down in a bowl of ice water to keep it firm.
Keep squeezing more milk out, as much as possible, until the liquid you squeeze out is as clear as possible. You can use paddles, if so inclined, but we prefer to use our hands.
Keep cooling in ice water in between squeezing more milk out.
Once you've gotten as much milk out as possible, add the sea salt on top of the butter.
Work the salt into the butter with your hands or with butter paddles.
You can just put your butter into a container, refrigerate it, and use it as is. Or you can form it into sticks or blocks, then refrigerate or freeze. We use butter stick molds for this.
Press the butter into the butter molds.
Then level the butter in the mold.
Cover and freeze the butter sticks for 3-4 hours.
Once the butter has sufficiently frozen, remove the sticks from the butter molds, and wrap each one individually in wax paper. Then store in a freezer bag or airtight container in the freezer 'til ready to use.
And that's it! Now you have a batch of homemade butter to enjoy.