To speed up the cooling of the milk, you can gently pour the milk from the saucepan into a sterilized container, cover it, and put it in the refridgerator or freezer. Regardless of how you are cooling the milk, monitor the temperature with a thermometer, until it reaches 110 degrees Farenheit (44 Celsius). Note that some glass thermometers cannot handle high heats and may break. A kitchen thermometer like the one shown on the right is most suited for this.

Once the critical temperature has been reached (this can take about 30 minutes) you are ready to add the yogurt starter culture; remember that you must use a yogurt with active or live cultures for this to work (a previous batch is perfect). You need about 1/4 cup (60mL) of starter culture for each quart (liter) of milk. You should warm the starter culture to 100 degrees F (44 Celsius) before incorporating it into the milk. Using the microwave is fine, but do not overheat the yogurt because you may kill the culture. When the appropriate temperature has been achieved (usually less than 30 seconds) gently stir it into the milk using a sterilized utensil. Thorough mixing is important but do not incorporate too much air into the liquid.