Pour your milk into the slow cooker. Place the lid. Turn the slow cooker on to the low setting and heat for 2.5 hours.
Once heated, unplug the slow cooker and allow it to sit for 3 hours. Remove the cup of yogurt you plan to use from the refrigerator and allow it to come to room temperature as the milk is cooling.
After the three hours, add the room temperature yogurt to your cooled-down milk. Stir it to combine all the yogurt. You do not want lumps. Replace the cover and wrap it in a towel or sit in a dark place where it will stay at room temperature. Let it set for 12 to 18 hours.