In a 4-quart saucepan, pour in the whole milk and place it over medium heat. Stir frequently until the milk reaches a temperature of 180-190°F, which usually takes about 10 minutes.

Once the milk has reached the desired temperature, remove the saucepan from heat and allow it to cool down to 110°F.

After the milk has cooled, whisk in the plain yogurt that contains live cultures until it's fully combined.

Transfer the milk and yogurt mixture into a clean glass or ceramic container and cover it with a lid or a clean towel. Place the container in an oven that’s turned off but with the light on, allowing it to rest undisturbed for 10-12 hours.

Once the fermentation period is complete and your yogurt has thickened, transfer it to the refrigerator for at least 4 hours.

After your yogurt has chilled, stir in your desired sweeteners or flavors such as honey, vanilla extract, or fresh fruits before serving.