Drill these onto your studs before you add tile or stone, or drywall.
Once your drywall, tile or stone goes up, you will have the steel posts sticking out into the room horizontally. Back Drill your mantle and slip the mantle onto the brackets for a “floating mantle” look.
If your drywall is already up, cut the drywall along the stud and sit the steel directly on the wood. Mud over the vertical part, and you will still have a seamless look.