To repair ceiling plaster, the loose plaster must be reattached to the wood laths by boring holes through the plaster and using an adhesive. Some sections of the loose plaster may be too loose and beyond repair. These sections have to be cut away and new plaster applied.
To judge which parts of the ceiling can be repaired and which parts need to be removed and new plaster applied, feel how much give there is in the loose sections by pressing it back into place. If there is more than half an inch of give, the plaster is likely too loose and there's too much debris accumulated to be reattached properly. Remove those sections carefully using a sharp utility knife.
The rest of the loose plaster ceiling needs to be reattached by boring holes into the plaster at a point that connects at the center of one of the laths. Use the exposed laths as a guide to where these holes should be and drill them every three to four inches apart on the loose plaster. For a proper connection, bore the hole completely through the plaster, stopping at the surface of the lath. Inject a latex or acrylic adhesive into the holes to reattach the plaster to the wood laths. Next, apply new plaster over the exposed laths and smooth out connections.