If you have never tried making your own yogurt perhaps this simple technique will make it easier for you to try for yourself.
yogurt, be sure this is living active culture yogurt
In a medium sized pot add the 4 cups of milk and start warming it over a medium flame. Attach your thermometer. If you are using an alarm model, set it to 181 (F). As soon as the milk reaches 181 (F) move the pot to a cold burner and let it cool down on it's own to 110 (F).
While the milk is cooling measure out the 2 tablespoons of live culture yogurt into a one cup measuring cup and set aside. Prep your crock pot by filling a quart jar with water and putting it into the crock pot. Fill around the quart jar with 110 (F) water until it is full. Remove the quart jar, turn your crock pot to Low for five minutes, then turn it off again, maintaining the 110 (F) water temperature using this method while the milk cools.
Right when your cooling milk reaches 110 (F) remove any skin that may have formed and then add about a cup of the warm milk to the waiting yogurt. Whisk very thoroughly, then pour this back into your pot of milk. Whisk a second time making sure that your culture is well incorporated into the milk.
Pour your milk mixture into a waiting quart jar and cap it. Put the capped quart jar into the crock pot water bath. Cover with tin foil and a few kitchen towels.
Turn on the crock pot once an hour to Low for five minutes, then back off again. Maintain the water temperature near 110 (F) throughout the setting process, which can take 2 - 4 hours. Begin checking the yogurt to see if it is set after 2 hours - mine normally takes 3 hours to be thick, well set, but still mild in flavor. If you prefer a tangier version of yogurt you can let it set for up to 8 hours safely.
Move the quart jar of yogurt to the refrigerator and enjoy! Be sure you save the last 2 tablespoons for your next batch.