Place drop cloths around the base of the fireplace.
Don goggles and work gloves to protect yourself, then insert a cold chisel into the mortar between the bricks and strike the end with a rubber mallet. Use this process to remove 1 inch of the mortar from the fireplace. Use your hands to feel the mortar after you have removed the initial layer. If the mortar is still weak, continuie removing the mortar until you reach solid mortar.
Brush out the spaces with a wire brush.
Fill a large plastic bucket with pre-mixed mortar powder and water in the ratio described on the package.
Integrate the water and mortar powder fully with a trowel until the mixture is uniform. The mortar should have the consistency of peanut butter.
Lift the mortar out of the bucket with a pointed trowel and pack it inside the joints in a ¼-inch thickness. Wait for 15 minutes while the mortar dries and then apply a second ¼-inch layer of mortar. Continue building up the mortar in ¼-inch layers until it is uniformly level with bricks of the fireplace. The mortar will cure within 24 hours. Do not use the fireplace until the mortar has cured.