Take the furniture out of the room and lay tarps over the whole floor.
Remove all loose paint or other material from the wall by scraping it with your 10-inch drywall knife. If there are cracks in the wall, fill them with plaster and spread it smooth with your knife.
Spread plaster in a thin coat (about 1/8-inch thick) over one upper corner of the wall with the drywall knife, covering as much area as you can comfortably from one position.
Scrape the plaster flatly over the surface with the front edge of the knife, holding it at a steep angle to get a paper-thin layer of it over the surface. The plaster should fill any dips or depressions on the drywall while being almost transparent over the flat parts.
Let the plaster dry for four to six hours.
Apply a second layer of plaster if necessary to get to the natural look you want. Let the second coat dry for four to six hours.
Gently sand the wall with your drywall sander, getting it smooth and flat. The wall is now ready for repainting.