Add milk to a clean, sterilized jar. Stir in starter.
Cover with a barrier such as a fabric scrap and set aside in a cupboard for 12 - 48 hours.
After fermentation, chill the yogurt in the fridge for 6 hours before eating. Store in the fridge for up to 1 week.
Optional for Greek-style yogurt: After fermentation, line a colander with cheesecloth or a nut-milk bag. Place the lined colander in a bowl large enough to keep the colander from touching the bottom. Add the yogurt to the colander and place it in the refrigerator to drain. I drain mine overnight.
For a thicker yogurt, replace half of the milk (2 cups) with half-n-half.