Buy a small container of heavy cream. Fill a jar halfway. Then shake! Takes about 8 minutes to separate. Drain off excess buttermilk. Use the same day. (If you want to store it, you need to rinse it more. Just add ice water, and drain. Repeat until water is clear, then you can store it in the fridge for a few weeks.) You can also use the buttermilk that you drained off to make biscuits!