Step 1:  Sterilize one or two 1 pint mason jars, depending on how much crème fraîche you want .  You can boil them in water, clean them with a bleach solution, or clean them with Star-San cleaner.

Step 2: Pour heavy cream into each jar, leaving a little head-room as shown below.  If you have a source for raw heavy cream you could try that at your own risk.  Pasteurized heavy cream would be great but is hard to find.  Ultra-Pasteurized Heavy Cream is the norm in grocery stores, and that is what I use.  Ultra pasteurized milk does not work for making cheese, so I was skeptical if it would work for crème fraîche but it did.   I will keep looking, but have so far been unable to locate regular pasteurized heavy cream.

Step 3:  Gently heat the cream to 85 to 90 degF.  You can use a water bath as shown below, in a pot, similar to pickling.  Don't use high heat.  Stop when the thermometer reads 83 or so and let it slowly rise so it doesn't pass 85 degrees by much.

Step 4: Remove from heat, leave in the water bath, and sprinkle 1/8 teaspoon of Mesophilic Aroma B Starter culture on the surface of each pint of warmed cream. Let the starter culture (which is a powder) hydrate on the surface of the cream for 5 minutes, then whisk it into the cream by hand.

Step 5: Lightly place the lid on the jar, and let sit at room temperature for 12 hours.
Step 6: Screw the lid on tight and let sit at room temperature for 24 hours.  Check it, and if it is nice and thick and can be scooped out with a spoon, it's done!   Store it in the refrigerator.