Take out the paper wrapper of the condensed milk. Place it on its side, in a pot big enough to cover the can until 2 inches height. Fill the pot with water until 2 inches above the can. Bring it to simmer, cover the pot and cook for 3 hours.
Check the water level every 30 minutes. The water level SHOULD NOT at any time, be below the can. Keep adding hot water as needed. After 3 hours, remove the can from the pot and leave it to COOL COMPLETELY. Do not open it while it's still hot/warm.
When the can has cooled completely, open it and enjoy!