Watch this video from This Old House to learn how to repair a plaster ceiling.
1. Screw plaster washers to ceiling along and around cracked area; be sure to drive screws into wood lath above.
2. Use trowel to spread joint compound onto ceiling, covering the repaired area.
3. Press insect screen into wet compound; smooth flat with trowel.
4. Allow joint compound to dry at least 24 hours.
5. Sand away high spots and ridges with 100-grit sandpaper; fill low spots with joint compound.
6. Apply another thin coat of compound, if necessary, let dry and sand smooth again.
7. Once repair is complete, apply one coat of primer followed by two topcoats of paint.