Grab a suitable-size pot and pour the milk into the pan.
Put the pot on medium flame and let it boil. When it boils, turn the flame down and let it simmer for 15 to 20 minutes until its extra water evaporates completely.
Pour it in a suitable container and let it cool down. The warmth should be as much as you can feel it with your finger when you measure it with the fingers.
When it reached the desired temperature, mix yogurt with one glass of milk and stir it until it gets smooth.
Add yogurt to the rest of the milk in the container little by little and stir it gently.
Cover the container with warm clothe and towel and keep it warm for 5-8 hours and never move it in this period.
After 5-8 hours, uncover it and refrigerate it for 1 hour until it chills completely.
Your Persian yogurt (Mast) is ready to use.