i use to make yogurt all the time by heating full fat milk until almost boiling with a slight milky froth/foam on top and turn off the heat, once it had cooled down a little i then added a teaspoon of natural yogurt north, east, south and west in the pan, covered it with a heavy lid and left it in the kitchen or airing cupboard until the next day where it has set beautifully, i then transferred the container as it is into the fridge and used it happily for the next 2-3 days
hth
Anna x