1/2 gallon whole milk slow cooker cheesecloth 1/2 cup plain (unflavored, unsweetened) yogurt with live cultures
Pour milk into a large slow cooker (crock pot). Cook on low setting for 2 and 1/2 hours, then turn off and let sit (covered) for 3 more hours. (Don't refrigerate - just leave as is.)
Sit in 1/2 cup yogurt, replace lid, then unplug the crockpot and carefully wrap the covered container in a large bath towel or an old, clean blanket. Make sure to cover the lid, as well, as you want to keep light out and keep the entire thing warm. Let sit (unrefrigerated) for about 8 hours.
For thick yogurt, line a colander or larger strainer with cheesecloth, pour in the yogurt, and let sit and strain for about 15 - 20 minutes. For Greek yogurt thickness - let strain for an hour.