This recipe is from Nourishing Traditions. It’s nice and basic. I’ve converted to metric. This tends to be a little thinner than commercial yoghurt. You will need a candy thermometer.
Stove Top Yoghurt
1 litre full cream milk
1/2 cup good quality yoghurt (when you get it going, save the last half cup of each batch to make more)
Heat milk to 85 degrees and allow to cool to 45 degrees. Stir in yoghurt and place in a shallow glass, enamel or stainless steel container. Cover and place in warm oven overnight. Transfer to refrigerator in the morning.
Of course I don’t use the oven, I use my easiyo thermo and fill the outer container with hot but not boiling water.