Summarize this article:

Grip the bottom of the lock with a pair of vise-grips. Tighten the nut on the handle to ensure the lock doesn’t move around while you’re holding it. Tilt the lock away from the object it’s attached to. Don’t use your bare hands to hold the lock since you could burn yourself. Pick any location on the shackle to heat. Light the torch with a striker and turn the valve on the nozzle to make the flames into a tight cone. Hold the tip of the flame against the shackle until it turns red hot.  Find an area on the shackle that you can easily maneuver a saw around. Work in a well-ventilated area away from flammable materials. Keep a fire extinguisher nearby in case there’s an accident. After the lock is red hot, turn off the propane torch and let the shackle cool. Don’t dip the lock in water to cool it down, since this could strengthen the steel again. Use a fine-toothed hacksaw meant for cutting through metal. Hold the lock in place with vise-grips in your non-dominant hand, and pull the back and forth across the area you heated with your dominant hand. Apply firm pressure to the lock to cut through it completely.  The heat softens the metal and helps the saw teeth stay in place as you try to cut the lock. Hacksaws work best on metal locks.

Summary:
Keep the lock sturdy with vise-grips. Hold a lit propane torch to the shackle for about 3 minutes. Let the lock cool down for 2-3 minutes. Saw through the area you heated.