This is probably the easiest way to make your own yogurt, but it definitely takes a bit longer. That is not a problem for me, but for both Kayte and Rachael it is because their families eat the yogurt up quickly, making it difficult to keep enough made at any given time.
The method I use doesn't require a thermometer, but you do need to pay attention to the size of your slow cooker, how much milk you are using and the timing of each step. The basic requirements are for the milk to reach about 180 degrees, then cool down to about 110 degrees before adding the yogurt starter, then the mixture needs to stay between 105 and 110 degrees for 8 to 12 hours.
I use a 6-quart slow cooker, 1/2 gallon of milk and 1/4 cup of yogurt with live cultures as a starter. Add the milk to the slow cooker and heat on low for 2 3/4 hours. At the end of that time, turn the slow cooker off and allow to cool for 3 hours, leaving it covered. When it is cooled (it should still be warm), remove 1 cup or so of the warm milk and add 1/4 cup of either store bought yogurt with live cultures or yogurt reserved from a previous batch (I try to have the starter close to room temperature so it doesn't cool the mixture down too much). Mix well, then pour back into the rest of the milk and mix well again. I like my yogurt sweetened, so this is when I add about 1/3 cup of sugar and about a tablespoon of vanilla extract. You can also add a vanilla bean or other flavoring, but don't add fruit or jam because it may make your yogurt watery. (You can also add any sweetener or flavorings at the end of the process.) Put the lid back on the slow cooker, wrap well in several heavy towels and allow to culture at room temperature for 8 - 12 hours. I like to wrap the slow cooker insert in towels and put it in the oven with the light on overnight. The yogurt is done after that culturing time (it should still be slightly warm), and is ready to go into the refrigerator to cool.
If you want Greek style yogurt or just want it a bit thicker, strain it through cheesecloth or a linen dish towel for a few hours - the longer you strain it, the thicker it will be. In the picture above, you can see that some parts end up thicker than other, but just stir it up good. I usually strain it after cooling, and about 4 hours is maximum for my yogurt. If you strain it too long, you end up with yogurt cheese, similar to cream cheese. Don't discard the whey that drains out - we did a previous post on many ways to use it.
One of my favorite ways to store the yogurt is to put it into 1/2 pint jars with a little fruit or jam at the bottom, mixing it up just before eating. Don't forget to set aside enough to start your next batch! I have had to go back to the store to buy enough for starter several times, especially if my grandkids help themselves to it.