Place milk in a large, heavy saucepan over medium heat. Heat, stirring often, until a thermometer inserted into the milk reads 82 C. Turn off heat and let milk cool, stirring often, until it reaches 46 C.
Pour about 1 ladle of warm milk from the saucepan into a bowl and whisk in yoghurt. Pour this mixture back into the saucepan and stir to combine. Ladle the liquid yogurt into clean glass jars and cover loosely with lids.
Place the jars in a warm spot in your kitchen, wrapped in a towel if your kitchen is breezy. Let incubate until yogurt thickens, 12 to 24 hours.