Boil milk in a large vessel that can accommodate overflow (just in case).
Allow it to cool completely. It won't sour if just left out for a few hours. Just leave it, run errands, have dinner, etc. until it cools completely. If it doesn't cool completely, the yogurt will be sour.
Take two tablespoons of good, regular, non-Greek yogurt, and mix it with the boiled milk until completely diluted.
Pour into small glass containers, or into a large pyrex (can be scooped out for smaller portions).
Place in oven at 100 degrees for 4 hours or overnight until set.