You don't need powders, packets and yoghurt machines to make your own plain yoghurt at home. All you need is milk, and some of your last batch of yoghurt.
1. Pour 2 litres* of your favourite top quality organic/biodynamic milk into a saucepan and bring to near-boil (bubbling) on the stove.
2. Allow the milk to cool a little, and pour into clean glass jars.
3. Allow the milk to cool further, until it is between hot and warm (it's a touch thing... by trial and error you'll work out the right heat** for optimum yoghurt...). Add a tablespoon of your last batch of yoghurt*** to each jar, and stir in.
4. Put lids on the jars, and put in a small cardboard box or carton filled with packing material and/or tea towels. Close up the box and put in a small cupboard that you won't open until next morning.
5. Next morning open cupboard, take jars of yoghurt out of box, refrigerate.
*This is how much I make at one go, but of course it's up to you how much you want to make.
**No doubt there is an optimum geeky temperature you could measure but I've never bothered.
***Or of your favourite bought plain yoghurt.