All you need to do is use heavy/ whipping cream- (unsweetened, of course), and beat it with a mixer, food processor, or blender till it turns into butter. For old-fashioned flavor, I would do my best to get organic milk.

It will first whip up into whipped cream, but if you keep whipping it, will suddenly turn from a creamy, whipped texture, into blobs of butter. Strain out the watery leftover liquid (true buttermilk). You can even process the cream by shaking it in a jar- but it would take much longer- about half an hour.

Then, you need to "wash" the butter to keep it from going rancid quickly. Do this by putting a cup or two of ice water in with the butter and beat or process it, then strain it off. Do this several times, until the water looks clear after being beaten with the butter. Finally, strain off all the water, and store. It can be salted or flavored with herbs, if you like.