It takes a skilled plasterer to be able to make good properly in order to not see any repairs.
The damaged area should carefully be removed, trying not to cause any damage to other areas.
Once the damaged area has been removed, brush the area down to get rid of any dust, then you should rake out a mortar joints to create a key. The background that is being repaired should then have a layer of PVA adhesive applied. While the area is damp you can apply the plaster/material to make the repair. Rule in the floating coat and key it to prepare for the finishing coat.