Heat the milk in a pot until it boils. Stirring frequently.
Once the milk boils, pull it off the stove and let it start to cool down.
Pull out about a ladle full and put it into a bowl to cool. You should also put your yogurt start out on the counter. Once the ladle portion and the yogurt start are room temperature, mix them together.
Wait for the pot of milk to cool to about 115 Degrees. Don’t have a thermometer? Just wait for it to cool until you can keep your little finger in the milk for 10 seconds without it burning you. (this is how I do it every time)
Once I have everything mixed together I put the mixture into a crockpot with a lid on because it holds heat better. I wrap the crockpot in a towel and blanket to keep it warm and then leave it safely on top of the fridge for 8-10 hours. (overnight is my favorite).
The longer you leave it out the more tart the yogurt is. You also want to leave it long enough to get thick. I find that 8-10 hours is the perfect timing for a mild and yet thick and creamy yogurt. If you want something more tart, leave it out to ferment longer.
Once it has fermented, without opening the lid of the crockpot, put it into the fridge for another 6-10 hours to cool before you eat it/put it into jars.