Add the milk to a large pot and heat it up on the stove, stirring constantly, just until it comes up to just before boiling point (about 180F), once you see the first sign of a bubble, you can remove from the heat.

Reserve 1 ladle of the hot milk in a bowl to cool down faster than the pot. At this time, also set aside 1 tsp of yogurt in another bowl to come to room temperature.

Then, cool down the pot of milk to 120F by placing it in an ice bath in the sink for about 10 minutes, or about 30 mins just sitting at room temp without the ice bath. You know it's cooled enough when you can withstand putting your pinky in it for 10 seconds, or use a thermometer. It should still be hot. Remove it from the ice bath.

Mix together the reserved yogurt starter into the reserved ladle of milk (doesn't need to be totally smooth), then add it to the pot of milk and whisk it in gently.

Cover the pot with a lid, then with a couple layers of towel or blanket. Set it aside somewhere draft-free where it won't be bothered or touched. Let it incubate untouched for anywhere between 8-30 hours, depending on the level of tang your looking for. The longer it incubates, the thicker it'll be and the more sour it'll be. I do 24 hours for a tangy yogurt used in Arabic cooking. After 24 hours, unravel the towel/blankets and transfer straight to the fridge for a minimum of 6 hours (preferably overnight) before scooping from it. Try not to open the lid so as not to introduce new bacteria into your yogurt.