Watch this video from This Old House to learn how to repair plaster molding.
1. Make cut through existing plaster molding with drywall saw.
2. Set sheet metal into cut and scribe molding profile onto the metal.
3. Use jigsaw, aviation snips and files to cut molding profile out of sheet metal.
4. Screw together two long 2x6s to create a plaster form; set form on sawhorses.
5. Screw wood blocks to metal template to create a sliding jig for 2x6 form.
6. Fill 2x6 form with wet plaster.
7. Slide the template through the plaster two or three times to create the exact molding profile; allow plaster to cure overnight.
8. Cut molding from form; trim to size.
9. Apply wet plaster to wall, and to the rear of the molding.
10. Press the molding to the wall, bedding it securely into the wet plaster.