The pictures show the preparation work (warming the milk, cooling and mixing in the yogurt) and the end result.
I tried to keep the temperature (should be constantly kept at 37°C or lukewarm for seven hours or more) with the oven method first, figuring S on a gas oven would be cool enough, what a mistake! It got too hot and was really difficult to keep at a constant temperature when I turned it off.
This time I kept it in a warm bath in a big pot. When cooking it was easy to keep the temperature right, leaving the pot next to the cooking food. After that I had to add some boiling water now and then from the kettle to the water in the pot, and it worked really well, mashaa Allah.