Heat the milk in a saucepan over medium-high heat until almost boiling, but not quite. Turn off stove and let milk cool down to a temperature where it can be easily touched, but is still warm.
Add the yogurt and stir to combine. Either store-bought or a few spoonfuls from your previous batch of homemade yogurt will work.
Transfer the milk into another container, if desired, and store in a warm (but not hot) place for 7-10 hours. I usually put my yogurt in a cooler containing a hot water bottle, but you can put it in a pot near an electric blanket, covered with towels for insulation. A thermos also will work, but the yogurt may be hard to remove once it thickens.
After seven hours, check to see if the yogurt has thickened. If it hasn’t thickened to your liking, let it sit another two to three hours and then transfer to the refrigerator. The longer the yogurt sits, the thicker and more tart it will be.
Enjoy your yogurt plain, or sweeten with sugar, vanilla, homemade jam, or honey.