Learn to make plain, custard, Greek, dairy-free, or lactose-free yogurt that is creamier than you can buy in stores, which you can also make sugar-free!
Add the store-bought yogurt and any flavorings you'd like to add to a small bowl. Whisk in about 1/2 cup of hot milk. Then, add that mixture back to the remaining hot milk.
Decrease the temperature to 86°F for 18 hours for lactose-free and until thick for custard-style, possibly as long as 6 hours. This step is known as culturing the yogurt. The longer you culture the yogurt the more acidic the flavor.
Check on whether the yogurt has set ever so often by jiggling the container a little. After about 2 hours into the 18 hours of culturing lactose-free yogurt, set aside a little while lactose still remains to use as a culture for next time instead of purchasing a store-bought brand. Label the culture "contains lactose". You can store it in the refrigerator for up to 2 weeks.
Once done, feel free to strain the yogurt using a fine-mesh or colander lined with either a coffee filter or 2 layers of cheesecloth. This step may be performed after chilling, if you desire.
To make Greek yogurt, double the store-bought yogurt and heat the milk to 205°F. Allow the milk to rest, covered, for 10 minutes. Combine the store-bought yogurt with some hot milk. Then, pour it back into the plain hot milk. Uncover and allow to cool down to 115°F. Culture at 110°F for 1 hour and then, 80°F until set.
*Small jars take longer to set because they cool quickly. So, feel free to use larger canning jars. Traditionally, 16-ounce jars take 3 - 4 hours to culture (if not making lactose-free yogurt.)
https://glutenfreerecipebox.com/yogurt-recipe-lactose-free-traditional-custard-or-greek/