Add the milk to the slow cooker insert. Cover and cook on low for 2 ½ hours.
Unplug or turn off the cooker and allow the milk to cool for 3 hours.
Stir in the yogurt. Cover and let it sit for an additional 8-12 hours. The yogurt should thicken over this time.
Transfer to an airtight container. Store in the refrigerator.
Notes
If you want thick Greek yogurt, you can strain the yogurt through a cheesecloth while it sits in the refrigerator.Reserve ½ cup of the yogurt to use when making your next batch.