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For the sour milk, you’ll need ½ cup (103 g) of sweetened condensed milk. Carefully pour it into a measuring cup to ensure that you get the right amount.  ½ cup (103 g) of sweetened condensed milk is about ¼ of a standard 14 ounce (397 g) can. Add the sweetened condensed milk to the measuring cup slowly. Because it’s so thick and sticky, it can be difficult to get the milk out if you add too much. Once you have the proper amount of sweetened condensed milk, add ½ cup (118 ml) of cold water and 1 tablespoon (15 ml) of white vinegar or lemon juice to the measuring cup. Mix the ingredients together well until they’re fully blended. When the milk is mixed with the water and acid, let the mixture sit for about 5 minutes. You’ll know that the sour milk is ready when you notice some curdled bits in it. You’ll make 1 cup (237 ml) of sour milk from the recipe.

Summary:
Measure the sweetened condensed milk. Stir in the water and the acid. Allow the mixture to stand for 5 minutes.