"You don't need a dedicated yogurt maker to make homemade Greek yogurt. If you've got a slow cooker, you're all set! The fat content of the milk doesn't matter. Use your personal preference, but make sure it is not ultra-pasteurized."
Pour milk into a slow cooker. Cover and cook on Low until hot, 2 1/2 to 3 hours.
Unplug slow cooker; keep lid on. Wrap slow cooker completely in a large, thick beach towel for insulation. Let sit for 3 to 4 hours.
Whisk plain yogurt together with 1 cup of hot milk from the slow cooker in a bowl. Return mixture to the slow cooker and stir to combine. Rewrap slow cooker with the towel. Allow to sit at least 8 hours to overnight.
Line a large colander with 4 layers of cheesecloth and place over a bowl. Ladle yogurt into the colander; let whey drain into the bowl below. Let sit for 30 minutes, so the yogurt comes out quite dry. Store yogurt in a covered container in the refrigerator for 7 to 10 days.