Eagle Brand’s site mentions these methods…

1 14-ounce can sweetened condensed milk (NOT EVAPORATED MILK)

OVEN METHOD: Pour 1 14-ounce can sweetened condensed milk into 9″ pie plate. Cover with aluminum foil; place in larger shallow pan. Fill larger pan with hot water. Bake at 425°F for 1 hour or until thick and caramel-colored. Beat until smooth.

STOVETOP METHOD: Pour 1 14-ounce can sweetened condensed milk into top of double boiler; place over boiling water. Over low heat, simmer 1 to 1-1/2 hours or until thick and caramel-colored, stirring occasionally. Beat until smooth.

MICROWAVE METHOD: Pour 1 14-ounce can sweetened condensed milk into 2-quart glass measuring cup. Cook on 50% power (medium) 4 minutes, stirring briskly every 2 minutes until smooth. Cook on 30% power (medium-low) 20 to 25 minutes or until very thick and caramel-colored, stirring briskly every 4 minutes during the first 16 minutes and every 2 minutes during the last 4 to 10 minutes.

“For safety reasons, heating the unopened can (an old cooking method) is NOT recommended.”

The warning about heating the unopened can is met with mixed reviews on the web.

Karyn of KarynForYou, boiled her can for 2 hours and it came out fine.   The warning is in place due to the risk of the can exploding.  I think it would be easiest to boil the can, however, it’s safest to use the oven or double boiler.

<– Karyn’s can of condensed milk boiling.