Put two or three inches of water in the bottom pan.
I use de-ionised water (car battery water) to stop my lovely pan from getting pitted inside by our hard water. I have topped it up twice over the past six months.
You may have to use a trivet to prevent an airlock between top and bottom.
Pour the cold milk into the top bowl and replace the lid. I have washed the already clean bowl again with hot soapy water, right before using it and kept it filled with clean boiling water for a few minutes to sterilize.
Raise the temperature over a medium to low gas flame and after a few minutes insert the probe of the thermometer. Watch it until the temperature reads 80 degrees Centigrade.
Take out the probe and switch off the gas. Let it sit for two minutes, stirring once halfway through to prevent a skin forming.
Even with the gas off, the temperature will creep up a few more degrees; probably up to 82 degrees C.
Converting units here!
Now we come to the place where the yoghurt will grow: almost a mini-oven inside an oven. The heat is provided by the pilot light only. The pan inside the oven has been warming there for a few hours. In my case it is a deep stewing pan with a teflon lining, 25cm diameter. I have measured the temperature inside the pan and it usually reads 41 degrees C. With the internal heat of the warm milk it probably goes a little higher.
Try to keep the oven and pan from cooling down by keeping the oven door closed and acting very quickly when you put the pan in and out.
You will need four sterilized glass jars of 1/2 Liter capacity each. Make sure they fit side by side into the oven pan. Put them inside the oven pan to get warm before you start heating the milk.
Whilst you are waiting for the milk to heat, put one rounded teaspoon of commercial yoghurt into each jar and then quickly put them back in the oven to stay warm. No lid needed.
The mixture of cultures I prefer is L.(=Lactobacillus) bulgaricus, L. acidophilus, L. Bifidobacteria, S.(=Streptococcus) thermophilis.
You may be fortunate enough to obtain these in tablet or powder form at your local healthfood shop. So far not available in Britain. That is why I use commercial yoghurts as a starter.
