Set your crock pot on low heat.
Put the 1/3 cup of yogurt on the counter so it will begin to come to room temperature.
Pour 1/2 gallon of milk into cooking pot and warm over medium heat on the stove top. Stir occasionally.
When the milk becomes bubbly and frothy, let it froth for another 5-10 minutes.
Take milk off heat and let cool until it is hot--but not burning--to the touch. Feel the inside of the crock pot; it should be warm but not burning to the touch. If it is too hot, turn heat setting down to "Keep Warm," or turn off crock pot completely to let it cool slightly.
When the milk is no longer burning to the touch, mix about 1 cup of milk with the 1/3 cup of yogurt, stirring gently. Pour the remaining milk into the crock pot, then gently stir in the yogurt mixture.
Turn off and unplug crock pot, cover with lid, and wrap in one or two heavy towels.
Let crock pot sit for at least 8 hours, or overnight.
After 8+ hours, place crock pot in fridge and chill completely.
After yogurt is thoroughly chilled, dip out 1/3 cup of yogurt and set aside to be the starter culture for your next batch.