Mix until well blended. Pour milk in a 1 qt. glass container (mayo jars work great). When temperature drops to 130 degrees, add 2 heaping T. yogurt. Do not stir. Cover tightly and wrap with a large towel. Or, what I do is put the container in a small covered drink cooler, and then you won't need the towel. Let sit undisturbed and in a draft-free area for 9 or more hours (up to 24 hours). Refrigerate for at least 6 hours.
Add fruit (if desired) when completely done. It is quite tart if you don't add any fruit, but we like it that way.
NOTES :This recipe is so easy, my 4-yr old insists on helping me! ;o)