Chip away cracked and loose mortar (wear goggles). Brush joints with fresh water (or mist/spray) to remove remaining dust and prevent the existing mortar from drawing all the moisture out of the new mortar. Apply mortar using pie shaped trowel, or caulking gun to smooth and make uniform, fill critical first and horizontal second.

Spray water on repaired area- can do for a couple days, Cover newly patched area for a couple days to aid curing. Meant to stretch/flex for a long seal, if it is cracked or brittle, there was not enough water on old brick or mortar during application.