Here's my method, based on how others on here make their yogurt:
I use a gallon of whole, raw milk, warmed it in a double-boiler (glass jar on a rack in a kettle of hot water), and held it at 185* for 20 minutes. I then quick cooled it (ran cold water into the kettle) to 95* and whisked a cup of milk into a cup of store-bought Greek yogurt. I then whisked another couple of cups of milk into the mixture, as it was still quite thick. Then I whisked it into the jar of milk. I removed a pint of the milk to a separate jar as my future culture, then added a cup of sugar and 1/4 teaspoon of vanilla to the big jar. Put a lid on both jars, set them in my Coleman cooler and filled it with hot tap water - about 115*.
Eight hours of innoculation time results in a very thick, only slightly tangy yogurt, just as I like it. As I am hooked on Greek yogurt, I am now draining it through cheesecloth.