I've made butter from cream (actually just last week), but I had no idea you could use spoiled cream!
He should have washed it several times until the water became clear, and then worked and pressed all the liquid out. Also he didn't mention adding salt (if you want salted butter), but otherwise this is just what the process looks like.
Sometimes I don't use up all our cream and have to toss some when it goes bad, but I WILL be trying this next time. Matter of fact, if it's just a little bit, I might freeze small batches of ruined cream until I have enough to deal with. I don't like waste!