Let's learn how to make the easiest fermented dairy product possible: Sour Cream. Truthfully, you can also call what you're about to make crème fraîche. Both items are essentially the same, varying only by the percent of butterfat involved.
Step 1: Gather High Quality Cream.
I like to use heavy cream instead of whipping cream as it tends to be richer in butterfat. I choose an organic as well as a local brand. You want a cream that is fresh and not ultra-pasteurized. Avoid creams that have extra ingredients (such as stabilizers).
Step 2: Pour Cream Into a Jar Then Warm to Room Temperature.
I use a wide-mouth glass Mason jar because I reliably have a lid that fits. Once the cream is in the jar, warm the cream to room temperature by 1) nesting the jar in a sink of warm water, 2) nesting the jar in a pot of warm water on the stove or 3) leaving the jar at room temperature for an hour or so. By 'room temperature', I very mean between 70 and 80°F.
Step 3: Add Cultured Buttermilk.
Next add a splash (~2 tablespoons) of cultured buttermilk to the room-temperature heavy cream then stir. I never measure the amount of buttermilk I add. More important is that the buttermilk is high quality and fresh--not a container that has been sitting in the back of your fridge for 17 months. Also make sure you can find the word 'cultured' somewhere on the buttermilk bottle or box.
Step 4: Hold at Room Temperature for 1-2 Days.
Cover the jar and place it in a quiet (and dark) corner. Leave it to ferment for 1-2 days or about 20 to 40 hours. More important than the amount of time you leave the cream is the way the cream seems when it is done. Look for a yogurty / sour smell. Look to see if the cream has thickened slightly. When you see these changes, it's time to move to the next step.
Step 5: Add in Salt Then Refrigerate.
Stir a modest amount of salt into the soured cream, about 1/4 tsp per 2 cups of cream or more if you like a saltier product. Cover the jar again and place in the refrigerator.
Step 6: Enjoy When Cream Becomes Super Thick.
Once the cream has been refrigerated for several hours or overnight, notice how it becomes extremely thick. Enjoy at this point.
Step 7: Pair With Everything.
Once you realize how delicious and easy this food is, you'll start to make it regularly. I like to use my sour cream on spicy tacos (obvious), on toast with honey (breakfast), baked potatoes (classic), mixed with powdered sugar for a cake frosting (novel) or on top of any meal that needs a little extra 'glue'. Homemade sour cream is an amazing (plus probiotic and enzymatically rich) food. Enjoy it!