How do you make cream sour?

Through a fermentation process.



A good example would be to find a recipe using sour cream; it will probably explain how to make the cream sour.



If a recipe calls for sour cream (or sour milk) add a few drops of lemon juice or vinegar and leave until it sours. You can also buy sour cream at the supermarket.
Leave it out or in the sun =)
If you need buttermilk for example and dont have any on hand you can add about a 1/2 tsp of vinegar or lemon juice to a cup of milk, but only use this on baked goods, such as pancakes, or brownies. My grandma taught me this.



Sour milk substitutes are not good if you plan on making things such as salad dressings, then you will need to go to the store and buy the real thing. I wouldn