Apply Stones to the Hearth
Follow the same process for the hearth.
As the last hearthstone (Image 1) goes into place, tap the stone with your fist to set it in the mortar.
Go back and fill in the gaps in the wall and the joints in the hearthstones. Then, just squeeze out a thin bead of mortar into the gaps between the stones.
Remember: If the hearth platform has an overhang, make sure it's less than 1/2" (Image 2). If not, the stone could break if someone stands on it or if a heavy object is placed on it.
To finish the job, use a tuck pointer handle to clean and smooth the joints between the stones and give it a good wipe with the whisk broom.