Apply mortar to the top of the bricks and between the bricks to create a smooth, even surface using a trowel. Allow the mortar to dry for 24 hours and apply a second coat. Allow the second coat of mortar to dry for 24 to 36 hours.
Install the corner pieces of river rock veneers first and work your way inward. Install the remaining stones working from the bottom up or the top down. Apply a smooth layer of mortar to the back of each stone with a trowel before setting them onto the fireplace. Press the stones firmly onto the fireplace to create a bond between the wet mortar, stone and dry mortar on the fireplace. Set the stones no more than 1 inch apart.
Mix masonry mix and sand to create a paste. Add about 2 quarts of the paste to a grout bag. Squeeze the grout bag, applying the paste between the joints in the veneer.
Wipe the surface of the stones and the masonry paste with a damp sponge while the paste still is wet, removing any paste from the stones and flattening the paste to give it a natural look.