Sand the area with medium grit sandpaper. Remove any loose particles around the crack.
Fill any small cracks with joint compound. Place drywall tape into the wet plaster for reinforcement.
Allow the plaster to dry. Sand the area smooth, and repeat with layers of joint compound or drywall mud until the ceiling is smooth.
Use a screwdriver to tighten and adjust the loose screw back into the wood. Hammer in nails that are loose.
Cover the area with a thin layer of joint compound. Use a putty knife to skim the surface of the ceiling when you apply the compound. Let the plaster dry.
Repeat the process, adding additional layers of plaster to the surface. Sand each layer with fine grit sanding paper, and wipe away the drywall dust with a damp cloth.