Place the milk in a heavy bottomed saucepan, with the candy thermometer placed on the side of the pan into the milk.
This allows you to watch the temperature of the milk as it gradually rises, and ensure that it does not exceed the desired temperature.
Heat the milk over a very low flame until it reaches 93°C (200ºF).
As the milk heats up, stir it occasionally.
Keep the pot partially covered.
It usually takes about 20 to 25 minutes for the milk to reach 93ºC (200ºF).
Cool the milk down to 45°C(114ºF); keep the pot partially covered and stir occasionally.
I usually place the pot in a sink full of cold water to speed up the cooling process.
Once the milk has cooled down to 45°C (114ºF), mix 200 mL of the milk with 1 cup of yogurt.
Pour this mixture back into the rest of the milk and stir until fully combined.
Ladle the milk into the sterilized glass jars and cover with lids.
Place the jars in an oven with the oven light turned on.
Leave the jars in the oven for 12 hours. I usually do this overnight.
After 12 hours you will see that the milk mixture has solidified and become yogurt. You may see some liquid (the whey) has separated from the yogurt and risen to the top. You can either stir this back into the yogurt or pour it off and discard.
Place the jars in the refrigerator.
The yogurt will remain good for about a week so be sure to only make the amount of yogurt that you expect to consume within a week.
Once you're about to finish off your homemade yogurt set aside a cup of it to use for a new yogurt batch.
I make yogurt in two liter batches but you can make anywhere from 1 to 4 L at a time with the initial 1 cup of store-bought yogurt.