Chisel away the soft mortar. Keep chiseling until you reach the strong mortar underneath. Remove any loose stones and clear away the debris with the wire brush. This will give the new mortar the best surface for good adhesion. Mix your new mortar according to the manufacturer's directions.
Most mortar is mixed to the consistency of oatmeal.
Dampen the repair area with water. This will help prevent the existing stonework from absorbing the water from the new mortar. The water content of the mortar will determine how well it cures, so you don't want it to evaporate too quickly. Apply new mortar to the area with the pointing trowel. Add as much mortar as you think you'll need.