Remove all loose particles of plaster around the crack. Rub your hand or a stiff bristle brush over the crack and the particles will fall off.
Mix up joint compound and fill in the crack completely using a flexible drywall taping knife.
Tape over the crack using the fiberglass tape. This will create a slightly flexible covering for the crack and protect the joint compound applied in the above step.
Apply a 1/16-inch thick layer of joint compound over the tape. Use the flexible drywall taping knife to create a smooth finish on the coating. Allow joint compound to dry half way, and apply a second coating.
Sand the surface to a smooth finish once the joint compound is completely dry. Apply an additional layer if necessary and sand to a smooth finish once dried. Paint the repaired area using a paint color that matches the wall.