Pour whipping cream into a clean jar.
Add buttermilk to cream.
Stir the cream and the buttermilk with a clean utensil.
Put a lid on the jar and leave the lid loose. Do not tighten the lid. If the jar you are using does not have a lid, another option is to cover the lid of the jar with a clean dish towel or new cheesecloth.
Place the jar in a warm spot in your kitchen between 68-78°F, out of direct sunlight, and allow the cream to culture for 12-24 hours until the desired tangy taste and thickness is achieved.
If you are using milk kefir to culture your cream, follow the same instructions above.
If you are using yogurt to culture your cream, you will need to place your jar in a warmer place that can maintain a temperature of approximately 110°F. This place may be in an oven that is turned off but has the electric light or pilot light turned on.
Refrigerate the sour cream in a jar with a tight-fitting lid. It should stay fresh for approximately 2 weeks.