Gather materials. You need a sealant gun, cleaning material, a putty knife, and water. Also, play it safe: wear gloves and goggles for extra protection.
Prepare surfaces. They must be clean (free of dust, grease, and other contaminants) and dry. However, you may need to moisten joints later on. Some Fireplace sealants are highly alkaline, surroundings have to be covered, hands and eyes must be protected. Always check the usage instructions and prepare surfaces accordingly.
Apply fireplace sealant. Load the fire sealant cartridge into the gun and cut the tip of the cartridge or application nozzle to the desired opening width. Slightly moisten the joints if indicated in the product information, then inject the fireproof sealant evenly into them. Even out the fireplace sealant immediately with a wet smoothing tool, if necessary.
Clean-up. Immediately remove wet stains with water. You must remove cured fireplace sealant mechanically, like using a sharp blade to cut it away.
Let it cure. Let your fire sealant dry according to the instructions.