Remove any paper labels from the condensed milk can if any. Then in a large sauce pan, place the condensed milk can horizontally and fill up water upto two inches above the can.
Now turn the heat on and once the water boils, let it simmer for two and half hours to three hours.
If at any point, the water level reduces, then add more boiling water.
Once done, remove the can and let it cool down before opening the can.
You will get a thick dulce de leche. If you want hardened and more dark dulce de leche, then cook the can for another extra hour.
Open the can only when cooled down. If the dulce de leche has hardened, just loosen it over a double boiler or a microwave in medium power.
You can store the dulce de leche in the refrigerator for a month and use it as you like.