Pour the condensed milk into a large mixing bowl.
Add some warm water (about 160 - 170 degrees F) and stir well.
Add the milk and plain yogurt to the bowl and continue to stir gently in one direction.
Pass the yogurt mixture through a sieve.
After straining, pour the mixture into the prepared yogurt jars. Close the lids securely.
Put the yogurt jars into a pot.
Pour warm water (about 100-120 degrees F) into the pot to create a water bath for the yogurt. The water should touch no higher than the jar necks.
Put the pot's lid on and cover it with a towel. Incubate the yogurt for 6 to 8 hours.
After incubating, take the yogurt jars out of the pot and wipe them dry.
Refrigerate the yogurt for 1 to 2 hours. Enjoy!