Pour the milk into an adequately sized pot and heat gently until milk boils.
When milk comes to a boil, remove from heat immediately. I have found that boiling the milk, rather than simply heating it to a lower temperature results in a much firmer texture.
Clip thermometer to the pot, and let the temperature come down to 110°.
As soon as this temperature is achieved, remove the “skin” from the top of the milk (any skin left will result in papery shreds in your finished yogurt), and whisk in the starter yogurt.
Pour the yogurt into the jars leaving at least half an inch of headspace and fit with lids.
Place the jars in your cooler and fill cooler to the top of the milk line with hot tap water. Your ideal incubation temperature is 110º, but as long as the water you put in your cooler is too hot to keep your hand in for more than a few seconds, you’re good. It will cool off before the milk gets too hot.
Snap the lid on the cooler and let incubate for 6-8 hours. It’s a good idea to replace the hot water half way through the incubation to facilitate the fermentation process. The longer it incubates, the more tart the taste, and many traditional cooking sources recommend incubating yogurt for up to 24 hours to “eat” all of the milk sugars and make sure enzymes are at their peak, so feel free to experiment with incubation times. We find the best flavor take about 8 hours.