Heat milk in large pot on medium-low to low heat, until temperature reaches 180 degrees (test with thermometer).
Once it reaches 180 degrees, take it off the heat and let stand until it cools to around 115 degrees.
Take two cups of the cooled milk and stir into the 4 tablespoons of yogurt to dissolve, then pour it back into the large pot and stir well.
Now divide the mixture into smaller glass containers, or keep it in the pot, and cover.
Place covered containers/pot in warm place for 8-12 hours. I put mine in the oven (don’t turn on the heat or anything) with the oven light on.
After this time, refrigerate the yogurt for 4 hours before straining it.
Now take the yogurt (which at this point is regular yogurt and you can eat it like this if you want) and put it in a yogurt maker/strainer or in 2 layers of cheese cloth over a large bowl and refrigerate for 8-24 hours. The more you refrigerate it while it’s straining the thicker it will get.