You will need:
1 cup heavy cream
1/4 cup sour cream or buttermilk
In a screw-top jar, combine the heavy cream and sour cream (or buttermilk). Shake the ingredients up a bit to thoroughly mix and let stand, covered, at room temperature for about 24 hours or until it becomes very thick. You can store your sour cream in the refrigerator in the original jar you mixed it in (or use some other resealable container) for up to one week. Make sure that your sour cream is well chilled before using.
If you want to make a lighter sour cream, substitute whole milk for 1/2 cup of the heavy cream.
PS: Buttermilk is made by adding 1 tbsp white vinegar or lemon juice to 1 cup of room-temperature whole milk. You then leave this to stand for about 10 minutes. Then use in the recipe as directed.