place new mantel on existing mantel
add shims and secure new mantel
trim top shims with a jigsaw
fasten crown molding to top of box wi
Secure the Mantel and Add Crown Molding
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.