It's made by reducing milk to remove some of the liquid and sugar is added.

If you're looking to make it so you can make it dairy-free, try HLA's coconut cajeta:
http://food52.com/recipes/9557_coconut_cajeta_chocolate_fondue
Just make the coconut part, leave out the chocolate. I've used it in place of sweetened condensed milk in puddings and it totally works. It actually tastes even better, in my opinion!

Actually, you don't (or, you don't need to). You buy it in the supermarket ready-made. Common brands are Carnation and Eagle. I suspect Whole Foods certainly has it somewhere. But if you really want to make it (though I don't know why you would), here is a recipe: http://bit.ly/xGKaUL