Remove with pliers any plaster anchors or other fasteners. Clean all holes in the drywall with a stiff brush. Fill a hole larger than 6 inches in diameter with drywall mesh or crumpled newspaper.
Mix drywall compound for small holes, or plaster of Paris for holes larger than 6 inches. Mix small quantities because either filling will dry quickly and be too stiff to work with.
Apply drywall compound or plaster of Paris into holes, no more than 1/2 inch at a time. Allow each layer to dry for 10 minutes before adding another layer. Fill the holes until the repair is flush with the wall surface.
Use a drywall taping knife and smooth a very thin layer of drywall compound or plaster of Paris over the repair. Allow to dry for one hour or longer.
Sand the repaired drywall. Prime and paint to match the wall, or repaint the entire wall.