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.