Lucia says, "I usually order 5 gallons of cream. It takes a few days to turn this much into butter, but if you don't have time, just freeze it for later. Cream separates when you freeze it, so it's fine for making butter (but not anything else).
"I culture the cream before I churn it, by letting it sit at room temperature for 3-5 days. You can put some yogurt in it if you want, but that's not necessary.
"We have done the churning several ways. I prefer the food processor, filled about half full, churned with a metal blade.
"It takes a few minutes. [You can hear the sound of the motor change, and you'll see grainy curds of butter separating from the liquid.] Pour off the buttermilk, fill with clean water, churn again. Toss the water, rinse until water is clear, and then take butter out of the processor a bit at a time. Squeeze out excess water in the butter with wooden paddle (rice paddle is fine) in a ceramic bowl. Pack into containers and freeze.
"The squeezing part is what takes the longest. If you don't squeeze out the water at all, it will turn it rancid, but it's tedious so I just do what I can and then freeze. If you use the butter quickly once you thaw it, it doesn't matter a whole lot really. I use plastic freezer containers. I think those plastic containers that Chinese food comes in are fine too.
"(Randy has churned all 5 gallons at once with a paint mixer. It works, but you have a five gallon bucket of butter you have to pour off and rinse. (!!!) I prefer small amounts in the food processor.)"
Sarabeth adds: I now skip Lucia's rinsing and squeezing steps (I've never actually had the butter go bad in the fridge, even after weeks), and instead, after pouring off the buttermilk, I dump the whole mass into a large bowl of cold water. I squeeze the butter under the water with my hands, forming it into one big lump, and then lift it out of the water for a final squeeze and pat-into-shape. If you chill the cultured cream before you churn it, you will get nearly perfect butter this way.