1 L milk
2 Tbs plain active yoghurt (look for the words probiotic or active cultures)
Heat your milk in a saucepan over medium heat to 82 C (almost boiling, this happens quickly). Remove pan from heat and allow milk to cool to 46C. This will take around 30 minutes. Once the milk reaches this temperature, stir in 2 tablespoons of yoghurt until combined. Now your job is to keep this concoction at 46C for the next 8 hours. There are a few ways to do this.
Pour your milk mixture into a mason jar (or similar) with a lid and place the entire jar into a small cooler filled with warm water. Leave for 8 hours.
Pour your milk mixture into a good thermos, wrap it in a towel and leave it for 8 hours.
Put your yoghurt in the oven if your oven has an oven setting as low as 46C and leave it for 8 hours.
After 8 hours you will have 4 cups of yoghurt that's probably thinner than what you're used to.
You can refrigerate it as is, or you can drain the liquid whey out by straining the yoghurt through cheesecloth for a couple of hours in the fridge. Just pour the yoghurt into cheesecloth, tie it up and hang it over a bowl. This will give you 2 cups of Greek style yoghurt.
Serve with fruit or add a little sweetness with a dollop of jam or squeeze of honey.
Credits: The Art Of Doing Stuff