You need to separate yogurt water. Pour the yogurt into a bag and close it tightly. Place a colander under it and place a pot under the colander to keep the water in the pot. Place a heavy thing on the yogurt bag so that the water is absorbed faster. village peoples in Iran use a piece of stone for this.
Let the water separate. What remains inside the bag is condensed yogurt.
After making the yogurt water, you should pour it into a pot and put it on a low heat. Wait until becomes thick and brown in color. It may take 2-3 hours or more.
Add some salt if you want.
Now turn off the heat and let it cool. Then put it in the refrigerator until it hardens and is ready to use.