Pour the entire gallon in a heavy bottomed pot or your crockpot. I used the pot and stood stirring so it wouldn't burn on the bottom until the temperature reached 180 degrees. I used a thermometer. It will be foamy at the edges and the entire top with become foamy. You want the milk hot and foamy, but not boiling.
Then I placed the pot in my sink with cold water to drop the temperature to 110-120.
At this point whisk in 3 Tablespoons of fresh plain yogurt with live cultures. There again, check the date to insure freshness.
I did not know this, but Greek Yogurt is just yogurt that has been strained. It has less whey and the protein becomes more concentrated per ounce by straining. There are lots of ways to strain yogurt. I lined a colander with wet paper towels. I wondered if this would work, but it did. The volume of yogurt was so much that it seemed to not be draining well. I removed some of it and put it in a fine screen colander by OXO. I was able to drain all of it well in about 30-40 minutes. I was worried about the paper towels getting in the yogurt, but the yogurt became cheesy against the paper and peeled right off. Some report using a thin white tee shirt to strain it works well, and others invest in an expensive chinois. The paper towel method worked just fine and if you had two colanders draining it would even be better for the volume. Next time, I may try the tee shirt.
This site is interesting and shows other cheeses and drinks that can be made at home. How about some homemade mozzarella?