Bring milk to just a boil. Make sure the milk doesn’t boil over. Remove the milk from the heat and allow it to cool to 110°F to 115°F (lukewarm).
Pour warm milk in a bowl and add 3 tbsp yogurt. Whisk them very well.
Take the remain (1 tbsp.) of yogurt and rub it all around the inside of the bowl where the edges of the milk touch the bowl. Yoghurt starts to form from the edges, so this helps to speed the process along.
Cover the bowl and set in a warm place for 7 hours (you can use a warm oven).