Place 4 cups of milk in a heavy based pot.
Heat milk to 80deg C.
Allow to cool to 40deg C.
Remove 1 cup of milk and add to one cup of quality store bought yoghurt.
Add that to the remaining 3 cups in the pot.
Place in a Wonderbag for the day or overnight.
Store in glass jar in the fridge.
Keep one cup in a separate jar in the fridge to be used to make your next batch.
You can obviously double this recipe. When choosing your store bought yoghurt, read the label to check that there are not loads of additives etc. You want as pure a yoghurt as possible. You can buy various yoghurt cultures, but I have found yoghurt does work well.
If you do not have a Wonderbag, please seriously consider getting one, and if you cannot afford one right now, let your loved ones know and maybe you can be treated to one for a birthday gift. If you can afford one right now, or even two, get one not just for yourself but for your domestic worker or a friend. They save so much electricity and make life easier knowing food can be safely cooking in your absence without leaving stoves or fires unattended. If you are still unsure as to the benefits of a Wonderbag, just visit their page to find out more.