Clamp a 2x4 to the existing mantel and pre-drill holes through both pieces of wood (Image 1). Drive in 2-1/2" screws to secure the 2x4 (Image 2).
Cut a 1x8 to length and add wood glue to the ends. Fasten the boards together with a finish nailer.
Place the new mantel on the existing mantel (Image 1), add shims (Image 2) and secure it with a finish nailer. Trim the top shims with a jigsaw (Image 3). The bottom ones can just snap off.
Mark a level line on the box and fasten the crown molding to the top of the box with finish nails (Image 4). Add wood glue to the end pieces for extra support.
The paint on the existing fireplace needs to be roughed up using wire brushes so that the mortar will adhere. Wipe off the dust with wet sponges.
Lay out stones to determine your desired layout. The stones in each box tend to be close to one another so it's best to mix the boxes.
Cut the stones using a wet saw with a segmented diamond blade (Image 1). Use a chipping hammer to rough up the edges.
Apply a layer of mortar to the fireplace with a trowel (Image 2). Wet the stones with a sponge, then put mortar on the back of the stones and set them in place (Image 3).