After 1 to 2 (or longedays, both the cream and milk are thickened from the acids produced by the proliferating natural organisms.
The cream has also conveniently risen to the top. I skim off the cream -- it's now sour cream -- and chill.
Then, I take the clabbered milk to the animals.
Voila, done! With hardly any effort at all!
To make sour cream without clabbering milk simultaneously, simply refrigerate milk until the cream has risen, skim off the cream, and let it sour at room temperature for 1 to 2 days as demonstrated above.