Lay your tarp below the affected area. Set up your ladder.
Scrape off and knock down any loose plaster and paint with a wide drywall knife. Leave only the plaster and paint that's solid.
Apply your premixed plaster to the affected area with the drywall knife, spreading it over the area and getting it smooth and flat. The plaster surface should be lower than the surrounding textured paint surface. Let the plaster dry overnight.
Sand the plaster lightly by hand, just enough to take off any ridges or high spots.
Paint over the affected area with sealing base primer, using your paint roller. Let it dry for eight hours.
Roll on textured paint over the area, completely covering it and connecting the newly painted area to the surrounding textured paint. Use a cement trowel to copy whatever contours are in the rest of the ceiling. Let it set for 24 hours.