Sour cream is a really easy milk product to make with raw cream. Don't fret though if you don't have raw cream because I will add in the couple extra steps you will need to take to make sour cream with pasteurized milk too.
Here is what you will need:
1 quart of cream
1/4 teaspoon aroma culture
Put cream into a mason jar, add the culture, shake, and leave on the counter for 36 to 48 hours. (If you only have pasteurized cream you will need to heat the cream to 75 degrees Fahrenheit before you add the culture - that way the good bacteria in the culture will be able to grow faster.)
At this point the sour cream will smell sour, but not have thickened.
Put the soured cream in the refrigerator and as soon as it is thoroughly cool, it will thicken.