Before We Start
Cultures: Most store bought sour creams have live cultures in them. We recommend getting a sour cream you like so the nuanced flavor is similar to it. You can also use buttermilk, but we think sour cream cultures makes the best sour cream.
Adjusting Sourness: If you want it to be less sour, 24 hrs is good. 56 hrs will be quite sour. 36 hrs is a happy medium and is the general timing we recommend.

Combine Ingredients
In a medium bowl or container, combine the sour cream and milk. Whisk together so there are no lumps. This will help distribute the cultures.
Next add your cream and whisk to combine. Transfer the mixture to a glass or plastic container that is air-tight. Place the lid on the container.

Refrigerate
Once it has thickened and developed the level of sourness you like, it is ready. You stop/slow the fermentation (souring) process by putting it in the fridge.
If there is some liquid collecting, you can spoon out that whey, it is also perfectly acceptable to leave it as is.