1. Heat the milk to 180°F while stirring intermittently. Remove from heat and cool (in the refrigerator is quickest, but on the countertop is OK, too) to between 115°F and 120°F.
2. Pour milk into 2 (1-quart) glass jars. Add 2 tablespoons of plain yogurt (either from a previous batch or store-bought) to each jar, and cap tightly.
3. Warm oven by turning to lowest temperature for 2 minutes, set a timer, then turn oven off. Place jars in oven with the light on—this should provide enough heat to keep the culturing yogurt between 95°F and 120°F (the ideal temperature range for microorganisms to multiply. Too hot and they die, too cold and they remain dormant). Wrap the jars in a towel for extra warmth, and leave in oven for 6–8 hours. Take care not to jostle jars during the culturing process. After that, add fruit or honey if you wish and transfer to refrigerator until ready to eat.
TIP: If you’re using raw milk (be sure to incorporate the cream) and want to keep your yogurt raw, heat the milk to 115°F and add the yogurt starter right away. Note that because you’re not starting with a sterilized product there will be unknown and prolific bacteria present, so you may have a less consistent (read: runny) end result.