Fill your jar half-way with cream.
Pop the lid on, and start shaking.
When the sloshing sounds stop, remove the lid, and check for whipped cream!
Pop the lid back on, and continue to shake until the mixture separates into buttermilk and butter.
Remove lump of butter (save the buttermilk for baking).
Rinse butter under cold water, kneading any buttermilk out of the butter.
Stir in a dash of salt if you wish.
Your butter won't keep for long because there are no preservatives. If you're not eating all of it the first day, store in the fridge for another day or two.
Store in the fridge if keeping for more than one day.