Heat up 4 litres of milk in a saucepan until it just reaches boiling point.
When it reaches boiling, remove from the heat and allow to cool until it reaches the temperature of 44 degrees (a digital food thermometer is really useful, but you can also just wait until the milk is cool enough for you to dip your finger in for 10 secs without scalding yourself).
Remove a cup full of milk and add to a separate bowl.
In to this bowl add your natural yoghurt and mix together.
Add this mixture back in to the saucepan of milk and stir until thoroughly combined.
Place a lid on the saucepan and leave at room temperature for 12 hours.
After 12 hours, put it in the fridge to cool for at least 4 hours.
You'll know it's ready when it's set, but can be spooned into a bowl.