In a jar, mix the cream and lemon juice (or vinegar) together, then pour in the milk. Screw a lid on tightly and shake well.
Remove the lid and put a clean piece of kitchen paper over the jar and secure it with a rubber band.
Leave it on the counter overnight (up to 24 hours) for it to set up.
After 24 hours it should be quite firm. Give it a stir and cover it with a jar lid. It will keep in the refrigerator for 2 weeks or so if it lasts that long!
Note: Because there are no artificial thickeners, your homemade sour cream may be thinner than commercial brands you may be used to. Have no fear, it’s fine. I’ve gotten used to the consistency and like it better since it blends in recipes so well.