Here are 2 safe ways to make this:
Stovetop method: Pour the sweetened condensed milk into the top of a double boiler. Bring water in the bottom of the double boiler to the boil; install the top, reduce the heat and simmer, uncovered, stirring occasionally. Check the water level in the bottom of the double boiler and add water if it gets low. Cook 1 to 1 1/2hours, until the milk is thick as pudding and caramel-colored (about the color of peanut butter). Remove the top from the double boiler and set caramel aside to cool, stirring occasionally.
Oven method: Preheat the oven to 425 degrees. Pour the sweetened condensed milk into a pie plate and cover snugly with aluminum foil. Put the pie plate in a large shallow pan and fill it with hot water halfway up the side of the pie plate. Bake 11/2to 2 hours, replenishing the water as necessary, until the milk is thick as pudding and caramel-colored (about the color of peanut butter). Set aside to cool, stirring occasionally.
and here's a
link to the recipe where I got the instructions.