1. Measure yogurt into a mixing bowl.  You want to use a volume of yogurt equal to 10% of the volume of milk; this works out to 3 tbsp of yogurt for every 1 pint of milk
   Fill mason jars with boiling water, to clean and heat them and leave uncovered
2. Place milk in the saucepan or stockpot. Heat over moderate heat, stirring and monitoring its temperature. When the milk reaches 180º, immediately remove the pan from the heat
3. Let milk cool to 115º (this may take awhile)
4. Empty the hot water out of the mason jars
5. Once the milk has reached 115º, add a little of it into the mason jar then add the yogurt, and shake until smooth. Then pour the rest of the milk into the mason jar
6. Screw the covers on
7. Put the jars all together in the place you plan to keep them warm, together with some extra (covered) mason jars full of hot water
8. In 8-10 hours, open one of the jars, and test it with a spoon. You should have yogurt. If you do not, try putting your jars in a half-jar-depth bath of hot tap water for a while, then putting them in a warm place again for another 8 hours

Note: This yogurt may not be as thick as the yogurt you are used to finding at the store. If you want thicker yogurt, hang your yogurt in cheesecloth over a bowl, and let it drain until it reaches the desired thickness. The liquid that drains into the bowl is called whey; you can use this whey as an ingredient in a Sauerkraut recipe, in recipes calling for buttermilk, or as a digestive tonic.