Pour milk into the slow cooker and cook on high for 2 - 2 1/2 hours.
Heat to 180 degrees. (Mine took 2 1/2 hours)
Turn off the slow cooker and allow milk to cool to 110 degrees (will take 1 1/2 - 2 1/2 hours. Mine took exactly 2 hours)
Allow the 1/2 cup of plain, unsweetened yogurt to come to room temperature while the milk is cooling.
Once the milk has reached 110- 120 degrees, mix in the yogurt into the milk and wisk gently. *Be sure milk is below 120 degrees or it will kill the live cultures.
Place the lid back on the slow cooker and wrap the slow cooker with several towels. Let it sit somewhere undisturbed so it can ferment for about 6-8 hours or 12 hours. It needs to look jello-like.
Then, remove the towels from the slow cooker and place it in the refrigerator for about 2 hours to cool completely. Below is what mine looked like after 8 hours of fermenting.
Do not shake or stir. It will set much better just left to cool. Below is what mine looked like after being in the refrigerator over night. From 9 pm until 7 am. You can see the whey that has risen to the top even more.
It is ready to eat at this point if you like. Just whisk until smooth and you are all set to add any fruit, honey, maple syrup, nuts and/or granola.
But, you can easily make it "Greek" yogurt by straining the whey from the yogurt in a very fine mesh strainer, like the chinois strainer I mentioned above, for about 30 minutes. I found mine at Homegoods for $10.00 :-)