Heat 1/2 gallon of whole milk on high heat.
Sanitize 1 quart glass jars.
Put 1 tablespoon of live culture (existing yogurt...I use Greek yogurt) in 1 quart jar.
Heat to about 190 degrees (or just before a simmer).
Let cool to about 130 degrees, or until you can put your finger in no longer than 2 seconds.
Stir occasionally to cool faster and prevent film from forming on surface.
Pour heated milk into jars and cap.
1/2 gallon makes 2 quarts...I drank some before I started :)
Incubate for 8 hours (I wrap jars in a blanket...)
(...and place blanket and jars in a big pot)
Let sit undisturbed for 8 hours, then refrigerate :)