Take a can of sweetened condensed milk, remove the wrapper.
Fill a large pot with water and place the sweetened condensed milk can in it. Water must be 1-2 inches above the can.
Cover the pot and boil the can for 3.5 hours. ( Set an alarm.)Protip: boil for 3 hours for soft dulce de leche and 4 hours for a firm dulce de leche.
Remove the can from water with the help of tongs. Or drain the hot water carefully and remove the can with thick towel.
Place the can on the counter and let it cool for at least four hours or preferably leave it overnight to cool.
Store the can in the fridge after opening.
Notes
Make sure the can is completely immersed in the water throughout the process. If the water evaporates add more water bring it to boil and then calculate the time.