There is a slightly kooky Greek-American who posted a how-to-make-Greek-yogurt video on You Tube that charmed my 3-year-old son and me. We decided to try his recipe. It took us a couple weeks to assemble the ingredients (cheese cloth, yogurt starter, whole milk....then more whole milk as we'd drink our supply). Finally, we got around to trying it. The majority of the time it just sits in your oven, proofing.
Sadly, our yogurt looked the same when we started as when we finished. Like milk.
I'm not sure what went wrong, but the most likely cause of error was heating to the wrong temp. I don't have a candy thermometer, so just guessed on when it reached 185 degrees, then cooled to 110 degrees. Better luck next time.
Here is an exciting photo of what the pan looked like sitting there in the oven for 14 hours: