Pour the milk into the slow cooker crock. Use High or Low setting to bring the milk up to 180° F. Keep an eye on it.
Once the milk has reached 180°, turn off the slow cooker and let the milk cool to between 110° F and 115° F. (this is the temperature that the yogurt cultures can live in, No hotter).
Add the yogurt starter to a small bowl and whisk some of the milk into it to temper it. Then whisk it into the milk in the crock.
Put the lid on and wrap it in a blanket or towels and let it sit in a warm spot in your kitchen. My favorite place to put it is in the oven with the oven light left on (the oven itself is OFF). This keeps it warm enough and is off of the counter. You will just put the crock with the lid in the oven, not the slow cooker housing. Don't wrap in blankets or towels if using your oven and oven light. No need! Also, if using the oven, write yourself a note that it is in there so you don't forget and accidentally turn it on!
Let the yogurt incubate for 8 to 12 hours. I like the 10 hour mark. The longer the yogurt incubates, the tangier it will be. Don't open the lid or the oven before the time is up.
When the time is up you should have some nice yogurt. Place the crock in the fridge, if possible, and let it chill about 2 hours before you stir it. This will help it stay thick.