Pre heat oven to 350.
Add 2L milk into pot. Bring milk to 180 deg F. (Keep stirring as to not burn the milk)
Just checking the temp.
Take milk off the stove, and bring the temperature down to 110F (You can use a cool bath like the picture)
Add 1/2 Cup (Active culture) yogurt.
Whisk everything together.
Place lid on pot and place into oven. Turn oven off and lend stand till ready.
Now if you wait 8 hrs you'll get a mild yogurt. You can wait 10 hrs for a medium. And 12 - 14hrs for a strong sour yogurt.
After a while you'll have yogurt.
Drain the yellow liquid in a strainer.
If you strain a bit. You will make plain yogurt. If you strain for 2hr you'll make Greek style yogurt. Also you can use your own yogurt as a starter this way you never have to purchase yogurt again.