Step #1 Heat the cream on the stovetop until slightly warmer than room temperature.
Step #2 (about 100 degrees with a thermometer).
Step #3 Remove from heat & whisk in the sour cream.
Step #4 Pour into a glass jar.
Step #5 Place the lid on top of the jar but don't tighten it.
Step #6 Put the jar in a warm room for at least 4 but up to 12 hrs.
Step #7 It will be thickened & you can use it then (especially if using as a sauce in a dish such as enchiladas), or put in the fridge for another 4 to 12 hrs to thicken even more.