It depends on why you're making the substitution. If you're out of sweetened condensed milk, you can make your own by adding 1 1/4 cup sugar to 1 cup evaporated milk and heating slowly over low heat until all the sugar is completely dissolved. Let it cool before using.
If you're looking for a lactose-free substitute, try combining 2 eggs, a cup of brown sugar, 1 tsp. vanilla, 2 Tbsp flour, 1/2 tsp baking powder and 1/4 tsp salt. Mix together until well blended and use as you would sweetened condensed milk.