Remove all furniture in the room and anything on the wall -- mirrors, pictures, paintings or whatever. Use a screwdriver to remove screws holding up the wall items.
Spread plastic sheets on the floor and any permanent fixture of the room that you cannot remove, including fireplaces, window frames and counters.
Wear thick gloves, goggles and a filter face mask to protect yourself from paint dust.
Scrape away all the flaking paint you can with a 6-inch scraper. Use a smaller flat-style scraper for harder-to-reach areas such as under windows and corners. Always use forward scraping motions when scraping the flaking paint from the walls.
Have an assistant hold the ladder for steady support when you're working to reach high spots.
Remove any paint the scraper did not take off with a heat gun. Turn the heat gun on and position the nozzle at least 6 inches from the surface of the wall, working in steady back-and-forth movements. Move to the next area when the paint begins to loosen and bubble. Be careful not to hold the heat gun in one position -- it can burn the wall.
Scrape the loosened and bubbled paint from the walls with the 6-inch scraper. Unplug the heat gun as soon as you finish using it, to avoid starting a fire.
Attach a medium-grit sanding pad to a handheld sanding block and sand the entire surface of the interior walls.
Wipe away all of the sanding dust with a large sponge dampened with clean water. Allow the walls to dry for one to two hours before doing anything else with the surface.