Take a clean glass/ plastic container, pour your heavy cream in it.
Add buttermilk into the heavy cream and mix it well.
If you dont get buttermilk from store, you can substitute it with apple cider vinegar or lemon juice.
Leave the bottle open on your kitchen counter for atleast 8 hours. You can keep outside for about 8-24 hours.
Cover the top of the jar with a cheese cloth so that the bugs dont end up in the jar.
Keep the jar in the refrigerator overnight, so the Crème Fraîche can thicken up. Once its thick, add dollops of it on anything you love :D This lasts in the fridge for about 2 weeks in an airtight container.