Put the condensed milk in an aluminum pot (enamel, it will burn).
Put the saucepan on the stove and turn the burner to medium heat. While cooking, periodically stir with a wooden spatula.
As soon as the milk starts to thicken, reduce heat to the minimum.
Then be guided by your own taste preferences. The longer you boil the condensed milk, the thicker and darker it becomes. But most importantly, don't forget to stir it, otherwise it will burn.
Cooking can be and in a microwave oven. To do this, put the condensed milk in a crock pot, cover with a lid. Put in oven and turn on medium heat. Set the timer for 40 minutes, at the expiration time check if the condensed milk will seem liquid, put be done to a turn.