1. Boil four 8-ounce jars on a rack inside a large pot for at least ten minutes to sterilize.
2. Pour 1 quart nonfat, low-fat, or whole milk (cow, soy, goat, or your choice) in a small saucepan. Affix a candy thermometer to the side of the pot. Heat milk to 180 degrees to kill all bacteria. Turn off heat and let milk cool to 105 degrees.
3. Stir in 1/4 cup premade plain yogurt or yogurt starter (available at natural foods stores). Pour mixture into jars and cap.
4. Keep the jars undisturbed at 100–110 degrees for four to six hours. There are different ways to accomplish this:
Pour 115-degree water into a Styrofoam cooler and place jars inside; cover.
Place yogurt jars in a large covered pot and put into a gas oven. The pilot light will keep them warm.
Put the jars in a clean sink with hot water; continue to add hot water as necessary.
5. Tilt each jar gently to determine whether yogurt has thickened to a creamy consistency. Refrigerate for up to one week. Save 1/4 cup of your new yogurt to use as a starter for the next batch.