Lay a drop cloth across your work space.
Put on goggles and long-sleeved clothing. You also will want to wear a respirator made for use with a painter's heat gun and have a fire extinguisher nearby.
Turn on the heat gun and aim it steadily at the wall about 2 inches from the wall surface. Allow the paint to blister. As soon as the paint begins to bubble up, it is time to scrape off the paint.
Place the gun on a new spot above the one you previously were warming. With your other hand, place your putty knife below the paint that has blistered and scrape upward along the wall.
Scrape the softened paint off the knife and into the paint can.
Repeat Steps 1 through 5 until the wall is clear of paint.