Compared with making whipped cream, it takes about twice as long for the butter to emerge. Save the buttermilk, it's delicious too! Once the bulk of the buttermilk is drained, it's important to wash any remaining buttermilk out of the new butter - it can go rancid if the butter is not refrigerated.
Up to two ingredients are needed: heavy whipping cream and optionally salt. If you own a cow, you're golden! But if not, cream bought at the supermarket works just fine.