Pour a gallon of milk into a large saucepan or Dutch oven and set it over moderate heat. With the candy thermometer in place, bring the milk to 180-185 degrees Fahrenheit to scald it, stirring occasionally with a wooden spoon to prevent burning. [approx. 45 minutes]

Remove the pan from the heat and let it until the milk has cooled back down to between 110 and 115 degrees. (Leave the candy thermometer in.) It will begin to form a skin on top as it cools. Don't stir, so as not to break up this skin and mix it into the milk, but you can swirl the pan a bit once or twice to help mix the milk underneath the skin. [approx. 1-1/2 hours]

While the milk is cooling, prepare your jars. Into each clean quart jar, measure 2 tablespoons of plain yogurt. (This is your starter. It's non-negotiable.) Also add honey, honey + vanilla, or stevia, as desired. Add half as much starter to the pint jar and leave it plain; you can use the contents of this smaller jar as starter for your next batch.

When the milk has cooled to the proper temperature, scoop off the skin with the wooden spoon and throw it out. Remove the candy thermometer, and ladle a ladleful or so of the warm milk into each jar. Put the lids on, and shake them well to distribute the starter and flavorings evenly. If you're using honey, continue to shake until the honey is dissolved. If necessary, wipe off the jars where some dribbled around the lids and down the sides.

Remove the lids and ladle in the remaining milk. Be sure to leave a little headspace in each one, so you can shake them again. Replace the lids and shake each jar one more time to mix the starter-filled milk from the bottom with all of the new milk you just added. Wipe down the jars again, if necessary (with something warm and wet if you're using honey).

Incubate the yogurt by holding it at 110-115 degrees for 6-8 hours.

When it's done incubating, just pop it in the fridge. :)