Lay tarps on the floor below the damaged ceiling. Set up your ladder.
Scrape the damaged area with your drywall knife, knocking down all loose plaster and textured paint. You should be left with just solid plaster around the indented area of missing plaster, with exposed lath. Scrap the surface around the damaged area, knocking off the textured paint for a few inches all around.
Press plaster into the damaged area, onto the lath, in a layer about ½ inch thick. Don't try to bring the damaged area out even with the ceiling, but get a solid covering of plaster over the lath. Let it set for eight hours.
Spread more plaster over the first layer, adding another ½ inch or so. Let it dry. Add another layer. Continue until you have built the damaged area out to the level of the surrounding plaster.
Sand the plaster smooth with a drywall sander. Coat it with base primer, using a paint roller.
Paint the whole repaired section with textured paint, using your roller.