How to make home-made butter?

Take double cream (you want a 48% milk fat, which is hard to get in the U.S. outside of a specialty market; heavy cream has a 30-40% milk fat content) and shake it. Forever. To be more specific, after sealing your double cream in, say, a jam jar, shake it until you hear the sloshing sound of butter forming (which will take anywhere from 15 minutes to an hour); drain the liquid off and then rinse the butter until the water runs clearly over it. Squeeze your butter (I use cheese cloth or a linen napkin for this) to rid it of excess liquid (excess liquid can lead to rancidity), shape it into a block, and wrap it with wax paper.

A traditional way that I learned while growing up is to do it in a jar, but it yields smaller quantities so you might have do it more often.

Basically, you fill a glass jar (canning jar or something similar) with heavy cream and put a small marble in it. You can use anything that will stay clean, it's just to provide agitation. Shake the jar for a while. After a little while you will see it start to form as the butterfat and the buttermilk separate. Continue shaking until you have a solid mass of butter. There will still be liquid, that will be buttermilk. The rest will be butter. You can add a pinch of salt if desired. Once you have a single solid mass of butter poor the buttermilk out. You can save it and use it for other recipes (pancakes, biscuits, etc). Using a pint of cream it took 10-15 minutes for the butter to form completely.

I'm sure there are other ways to do the agitation. I mixer on low speed will probably do the trick but you'll obviously need to watch out for splash.

If you have a Kitchenaid stand mixer (or I suppose any kind of stand mixer) with a "paddle" attachment (like the "K" thing in a Kitchenaid; something that's not like a whisk), you can make butter in that at the lowest speed. It'll splash around at first of course.

In my experience, butter made with store-bought heavy cream doesn't taste as special as you might think. Good-quality "cultured" butter, if you can find it, tastes a lot more buttery.

Now if you can get double cream from a local dairy (which in the US may be questionably legal, if you care about such things), then I'm sure it's possible to make really good butter.

Go to see the farmer and tell him to sell you some cream extracted from fresh milk.

At home, shake it with a kitchen robot, as long as it needs, but at some time, the texture will change to butter. You may add some salt or not.

That's the traditional fashion to make the best authentic tasty butter. Anything passed by any factory will never taste like that.

Be sure to process in a cool area, and keep your butter in fridge : it won't last as long as the supermarket one.