I have a temperature control on my slow cooker which makes the process easier as well. First you heat up the milk to 180 degrees. This kills all the other bacteria in the milk so the yogurt bacteria can grow. Unplug the slow cooker and check the temperature after 1.5 hours and then every 10-30 minute until its between 110 and 120. This is the ideal temperature for the yogurt bacteria to grow. Anything over 120 will kill that bacteria as well.
Mix in 1/2 cup of yogurt for every 8 cups of milk you used and then wrap the bad boy up.
It needs to be completely dark, warm and unmoved for at least 12 hours. I usually leave it 14-16. The longer it sits the tangier it gets....and I likes it tangy.
The first time I made yogurt I just bought a small pod of yogurt to use as a starter but since then I just use the last of the yogurt I made the week before.
And there you have it! Yogurt. Cheap cheap delicious yogurt. Initially the yogurt will have a lot of liquid on it...you can either stir it back in to make a more drinkable consistency or (what I do) spoon it off so the yogurt is thicker.
If you strain it with cheese cloth for a couple of hours you can get it to the thickness of Greek yogurt, if you prefer.
Favorite money saving trick ever.