Turn your crockpot on the low setting for at least 2 to 2 1/2 hours to be sure that it is warm.
Turn it off and let it set for 3 hours. Should be cooled to around 110 degrees F. Be sure to use your thermometer to check and be sure that it is at 110 degrees. It is important that it stays at this temperature.
Add 4 cups of milk that has been heated to 180 degrees F and cooled back down to 110 degrees F. Mix in 3 T of culture.
Optional: To be sure that your yogurt is thick you can add 1/2 cup of powdered milk. This is not something that I do but have heard others that have tried it. If your yogurt turns out too runny for your taste you can always strain it through some cheesecloth. Be sure to save your whey that comes out to use in your cooking.
Wrap your crockpot with a blanket or towel and let it sit on the counter for at least 8 hours.