I make my own yogurt every week. There are lost of reasons why- it cuts the cost to less than $2 for 2 litres, one less thing I have to get from the supermarket and it tastes amazing. Because its so cheap I don't feel as bad using it in cooking and baking.
I use powdered milk, it's always in my cupboard so I can make it whenever. My method is pretty simple. I use my thermomix now but use to do it in a saucepan.
1. Whisk your powdered milk at a ration of 1 c milk powder to 3c water(check the instructions on the packet. I make up 2 litters.
2. Stir in 1/4 c of your favourite Greek yogurt.
3. Heat slowly to 37 degrees stirring off and on, hold at 37 degrees for 20 minutes.
4. In the mean time clean the jar or container you are going to use.
5. Pour the milk yogurt mixture into your jar, wrap with a tea towel and put in a warm spot for at least 8hrs. Check and taste your yogurt- if its good pop into the fridge. Remember to save enough yogurt for your next batch.