To sterilise the jars place in oven at 100c for 10 min, turn off the oven and leave jars to keep warm until you need them. Wash and dry the lids.
Bring the milk to just below boiling point and hold for 3 mins, (95c if you have a thermometer).
Stand the hot pan of milk in a sink of cold water to cool. Stir to assist cooling until it reaches blood temp, (40c with thermometer), or until you can comfortably hold your finger in it for more than a few seconds.
Now stir in the natural yoghurt. If its too hot when you add the yoghurt it will kill the bacteria and won’t work.
Carefully pour the mixture into the warm jars and place the lids on.
Place in insulated bag wrapped with tea towels and hot water bottle to keep warm overnight.
After sitting either overnight or 6 hours in the oven the yoghurt will have formed. Place in the fridge to cool, before adding your own toppings to serve.