Sour cream is simply...............soured cream. Not rocket science to make, it can be made either from thick cream or from milk.
First you need to clabber your milk. I do this by taking my raw milk either directly from the cow (after straining to remove any impurities) or from milk that has been refrigerated. You can use milk with or without the cream. I pour the milk into stainless steel bowls and then cover loosely and allow to sit until it clabbers. Sometimes this takes 24 hours and sometimes, if the weather is cooler, it takes up to three days. You can view pictures of "clabbered milk" here.
After your milk has clabbered, take the curds and place them in a cheese cloth. Tie up the ends of the cheese cloth and hang somewhere to drain until the whey has separated from the clabber. When the whey as separated after several hours what remains is the sour cream. (The longer you let the cream drip, the thicker the sour cream will become. You can do this in the refrigerator is you do not want your sour cream to become as tart.)
Take the sour cream and mix in salt to taste whisking with a wire whisk until smooth.