To repair an edge you must remove all the plaster until you get a firm layer, cleaned up well from dust and debris.

Then attach a rod on a corner face as a guide to rebuild.

Fill the missing part with lime mortar.

Once dry, remove the bar and attach it to the other side of the corner and repeat the operation.

Once dry, smooth the edge with a wooden pad on which the sandpaper is mounted until the flat faces of the corner are perfectly made.