Create a double-boiler from two stock pots, with water in the outer one, and your half gallon of milk in the inner one. Using a thermometer (clip-on is ideal), heat milk to 185F, stirring often to prevent burning/sticking.
Remove pot of milk to a cold water bath in your sink until it lowers to 110F, stirring occasionally.
Stir in 2-3 Tbs of plain yogurt with active cultures, as a starter. Dannon is a good choice. For future batches you will use your own yogurt.
Cover and place on atop a heating pad, on a cutting board. Turn heating pad to medium and cover with a tea towel. Let it sit for seven hours undisturbed.
After seven hours, uncover and stir thoroughly. Pour into old yogurt containers, or similar, and place it in your refrigerator overnight.
After refrigerating overnight, your homemade yogurt is ready. It will last 2-3 weeks in the refrigerator. Be sure to save 2-3 Tbs for your next batch.