Put whole cream into a blender, food processor, or mixer, and whip it on high speed. It will initially whip up into whipped cream. Then the whipped cream will get thicker and thicker, like whipped butter. Don't stop here though!
After 2-10 minutes, depending on the speed and strength of your machine, there will be a sudden change when the milk fat and solids separate out dramatically, leaving thin liquid behind. Beware of this, if you're using an open mixer; if you're mixing at high speeds, suddenly liquid will appear from nowhere and start spitting out of the bowl. If you're using a food processor you'll hear a "slap slap slap" as the butter suddenly forms up and whizzes through the thin buttermilk.
After this, it's very important to drain away all of that buttermilk (which is really just skim, watery milk) and squeeze the butter under running water until all traces of milk have drained away and the water runs clear. Otherwise that leftover milk will cause your butter to go rancid within a few days. Squeeze it as tight as possible and rinse it well. Then pack it into an airtight container or into some plastic wrap, and freeze.
That's all there is to it! I just made two quarts of cream into a pound of good butter, using my Kitchen Aid. It took about 15 minutes, and cream that was at the end of its shelf life has a new use