Brush crumbling and broken mortar from between bricks with a wire brush. This clears the area of debris and prepares the surface of the fireplace for new mortar. Use a mask to avoid inhalation of mortar dust and goggles prevent dust from entering the eyes.
Prepare mortar with water following the direction on the back of the packet. Although reasonably inexpensive fireplace mortar is a specialist product often only available at masonry supply stores. Be sure the mortar you choose is specifically for use with fireplaces or high heat zones.
Place a small amount of mixed mortar onto a masonry trowel. Be sure to choose a trowel size that works for you. Often, standard size trowels can be difficult for people with small hands.
Smear the mortar into the cracks between fireplace bricks by pushing the loaded trowel across the area. It doesn't have to be perfect; the idea is simply to get as much mortar as possible into the crack to fill it completely. More than one pass over with a loaded trowel could be necessary.
Remove excess mortar from the surface of the fireplace using a damp cloth. This will tidy up the area, smooth the surface of the repair and push mortar even deeper into the cracks.
Allow the mortar to dry completely in accordance with the directions on the packet.
Use a paintbrush to brush away any excess debris from the surface of the new mortar.