Make sure all food-contact equipment, tools, and containers are cleaned and sterilized to avoid yeast or mold during incubation.
Pour milk into the heating/scalding pot or equipment.
(Optional) Add thickener (agar or similar) and sugar if using. Mix well.
Heat Milk gradually to 185F or above but not exceeding 200F. Stir in between to prevent scorching and curdling.
When milk reach the high temperature above, reduce heat and keep on stirring. Try to prolong this heating period and maintaining the milk's temperature in between 185F-200F limits for 10-30 minutes or as long as you can.
Remove milk from heat source and let to cool down to 110F or per cultures' optimum. Note that you can speed up the cool down by soaking the whole milk pot into cold water bath.
While waiting for the milk to cool down to culture temperature, prepare the incubating zone/equipment based on what you have. (See methods of incubating yogurt above).
Take some of the warm milk (at culture temperature) to dilute and mix with the yogurt starter. Then pour this mixture into the warm milk and mix them good.
Ladle the mixed warm milk into yogurt container(s) and incubate for 6 hours or more. (See How long to incubate yogurt).
Once the time is done, stop the fermenting process by chilling yogurt for 6-24 hours. Yogurt will be firmer and thicker the longer as it's chilled.