Prepare the area as you would for a repair to the flat surface. If there is an angled mortar fillet, carry out any necessary repairs to damaged areas using a mix of sand and cement, having removed any loose sections.
Using a proprietary repair kit, apply the adhesive to the affected area and allow to set in accordance with the instructions on the pack.
Cut tape to length and apply to the area. Smooth down with an old rag ensuring that the edges are well sealed.