Pour milk (I use whole milk) into your crockpot, add the lid and turn it on low. Allow the milk to heat for 2.5 hours. Don't forget to set a timer.
Once the 2.5 hours are up, turn the crockpot off and allow the milk to cool for 3 hours. Again, be sure to set a timer.
After 3 hours, add in the probiotic yogurt (either plain from the store like this, or from a previous batch of homemade yogurt). Be sure to stir thoroughly.
Place the crockpot with the lid on into the oven, close the door and turn on the oven light. Allow the yogurt to sit in the oven a minimum of 8 hours, or up to 24 hours. The longer you let it ferment, the thicker it will be.
Once you've allowed the yogurt to ferment, remove the crockpot from the oven and strain the whey (the liquid that accumulates on top).
At this point, you can add in any sweeteners you'd like, or leave it plain. Place in the refrigerator to cool completely and enjoy!
Notes
If you're starting from scratch, use a plain yogurt that has live cultures in it. Often times, it will say probiotic on the packaging and then list the live cultures in the ingredients section. The probiotic part is important because this is what is going to turn your milk into yogurt.
Prior to placing your milk and yogurt mixture into the oven to ferment, turn your oven on the lowest temperature (for me that's 170 degrees) and then shut it off. Then turn the oven light on to maintain warmth in the oven.
To strain the whey off your yogurt, either take a ladle or spoon and get as much as you can out, or line a strainer with coffee filters and drain it away.
Once you've made one batch of homemade yogurt, save 1/2 cup (if making 1/2 gallon) to use for the next time. Be sure to reserve the yogurt prior to adding any sweetener or other ingredients.
If you're making a gallon of yogurt use anywhere between 3/4 cup and 1 cup of yogurt as a starter.
Liquid sweeteners are best for this yogurt. I like to add about 1/2 cup of maple syrup, but you can use honey (just be sure not to use for kids under 1 year of age), or any other liquid sweetener. Or leave it plain if you plan on using it for something savory.