I use sweetened condensed milk to make flan. My grandmother, who was born and raised in Mexico, then came to the US and opened several restaurants, taught me. My family loves it so much, I not only have to serve it at every family dinner, I have to make several extras for them to take home!
First, heat about a quarter cup of sugar in a skillet until it melts and is a caramel brown color. Immediately pour it into a baking dish and swirl the pan to coat most, if not all of the bottom. (I actually skip this step when I'm making flan just for myself, because although I do like the custard itself, I'm not a fan of caramelized sugar. But, for the family, I have to make it just like "abuela" (grandmother), or I'd have a mutiny, lol.)
Pour one can of sweetened condensed milk into the blender. Add a teaspoon each of ground cinnamon and vanilla. Using the empty sweetened condensed milk can to measure, add 1 can each of milk and eggs (usually between 4-7 eggs, depending on the size of the eggs).
Pour into the caramel coated baking pan, cover with tin foil, and either bake at 350 in a water bath, or simmer on the cooktop in a double boiler, for about an hour. Chill and serve.