1 quart (946 mL) milk (any kind but if you use "ultra-high pasteurized" or "UHP" or "UHT" then you can skip step one, as the milk has already been heated to this temperature before the pack was sealed)
1/4 to 1/2 cup non-fat dry milk (optional)
1 tablespoons white sugar to feed the bacteria
pinch salt (optional)
2 tablespoons existing yogurt with live cultures (or you can use freeze-dried bacteria instead)