Add milk; cover & cook on low for 2-1/2 hours to 3 hours.
Unplug crock-pot. Keep lid on, and completely wrap crock-pot in large, thick beach towel, for insulation, and let sit for 3-4 hours.
At the end of 3-4 hours, in a small bowl, whisk plain yogurt with one cup of the milk from the crockpot. Return it to the crockpot, stir.
Keeping crockpot unplugged, completely re-wrap in heavy towel.
Allow to sit for at least 8 hours or overnight.
Place a large colander over a bowl. Line colander with flattened basket-shaped coffee filters and/or four layers of cheesecloth. Ladle yogurt into colander. Let liquid whey drain into bowl below. (If you'd like, you can save the whey for another purpose, like homemade ricotta cheese.) Let it sit for about 30 minutes or more, so it comes out quite dry.
Store in covered container and refrigerate. Will stay fresh for 7-10 days.
In a non-metal container, set aside ½-cup as your starter for your next batch.