To replace missing or broken bricks, take out the broken bricks, and clean out all the damaged mortar. Wet all the areas around the location of the new brick, apply mortar to those surfaces and the replacement brick, and insert the brick in the opening. Wipe off all the excess mortar that squeezed out of the holes.
If you are replacing fire bricks on the inside of a fireplace or in another location with bad lighting, use a shop light to illuminate the area. To conduct a thorough repair, examine the entire firebox to find other broken or loose firebrick, as well as loose, crumbling or missing mortar. Use a stiff-bristled brush or cleaner on the surfaces to find all the damaged spots. Once you find all the spots you need to repair and take out the bricks, clean out the surrounding areas using a stone chisel and stiff-bristled brush. If there are spots where mortar is crumbling, clean them out to be 3/4 inch deep.
Mix the refractory mortar for the repair according to the manufacturer's instructions. Make sure to wet the surface of these cavities before filling them with new mortar. Once you have filled all the cavities, smooth their surfaces out using a jointing tool. The mortar needs to harden slightly before you can do this. If there is mortar on brick faces, quickly remove it with a damp sponge before it hardens.