Heat the milk to a temperature of 180 degrees, and then let it cool to 110-115 degrees. You can do this by heating it on the stove top and monitoring the temperature with a thermometer, or by using a slow cooker. If you use the slow cooker, heat it on low for 2 hours and then turn it off and leave it covered in the slow cooker for another 3 hours.

Set up a cooler with a bath towel inside.

Fill a pot with a few cups of boiling water and place it in the middle of the cooler. Be careful to keep the pot from touching the sides of the cooler (the towel is a buffer). Cover the cooler.

Pour the milk in a large glass container, and add the yogurt. Stir to combine.

Place the glass container in the pot of water in the cooler. Place a meat thermometer in the cooler and close the cover. Ideally, the temperature will be between 90 degrees and 120 degrees. Let it sit for 6 hours.

After 6 hours, replace the water with a fresh pot of boiling water. Let it sit for another six hours.

After 12 hours, the yogurt should be done. If it doesn't seem ready, replace the boiling water again and leave it for another 6 hours. There will be liquid on the top (whey) and yogurt at the bottom. When the yogurt is ready, put the yogurt in the freezer for 20 minutes to help it thicken.

Separate the whey for another use by placing the yogurt in a strainer lined with a cheese cloth. Let the whey drip out until you have the consistency you want. I use this Butter Muslin, which is woven more tightly than some cheese cloth and it’s washable and reusable.