Sterilizing the Jars: Fill a large stainless steel pot with two inches of water. Place 4 pint glass Mason or canning jars, plus their lids into pot. Cover and bring water to a boil over high heat for about 8 minutes. (If you have a good dishwasher, you may opt to forgo sterilizing your jars as long as they are newly cleaned.) Alternatively, you can fill the pot with water, submerging the jars, and boil for 10 minutes. Place the pot, still covered, off the heat, until ready to use the jars.
Scald the Milk: In a large heavy-bottom stockpot, add the milk and warm over moderately-high heat, until it reaches 190°F/88°C. Take off the heat.
Cool the Milk: Let the milk cool on its own or if you’re pressed for time, fill your sink halfway with ice cubes and cold water then place the pot into the sink. Let the milk cool to 120°F/49°C.
Add Culture: Whisk the yogurt into the cooled milk until incorporated.
Drain Whey: Line a colander with cheesecloth or a fine-cotton tea towel and set inside a large bowl. Add the yogurt and let sit, draining, for at least two hours or until it’s reached the consistency you like. The longer it sits, the thicker it becomes.
Transfer the Milk: Pour the milk into jars (you can use a funnel if you’re worried about spilling), and seal with lids.
Cooling the Yogurt: Place them into a cooler and pour warm water (about 120°F/49°C) into the cooler so that it comes about ¾ of the way up the containers, and leave for 3 hours. Or, leave the yogurts in a warm spot on the counter with a tea towel for at least six hours or overnight.
Refrigerate: Place in the refrigerator to cool and set for at least 3 hours or overnight.