Heat 1 quart milk (cow's, goat's, soy, or rice) slowly to near boiling, about 185 degrees. Stir frequently to prevent scalding.
Cool milk to about 112 degrees.
Add 2-3 tablespoons room-temperature plain yogurt, or one packet of freeze-dried yogurt starter, and stir. For thicker yogurt, add ¼-½ cup nonfat powdered milk (optional).
Incubate yogurt at 108-112 degrees for four to eight hours. Use a towel-covered heating pad set on medium (or an electric yogurt maker) to keep temperature steady. The longer it sits, the firmer and tangier the yogurt becomes.
Stir yogurt and pour into sterilized glass jars.
Refrigerate for 12 hours to stop acid development; it will keep for a week refrigerated. Eat plain or add your favorite fruit or sweetener. Then use some of the homemade yogurt as starter for your next batch.