Heat milk to almost boiling and let cool to lukewarm. Mix in yogurt and pour into a wide-mouthed thermos (or several if they are small). Put lid on. Let set 4 to 6 hours. This is practically foolproof since the temperature is controlled for you.
If, after refrigeration, there is a little water (whey) on top, don't worry. You haven't done anything wrong. Just mix it in or pour it off and save for other recipes since the whey is high in B12 and minerals.
