Lay the shadow box moldings on two sawhorses. Paint the molding as desired. Let the paint dry completely.
Determine a layout for the boxes. Measure the wall area beneath the chair rail molding and above the baseboard molding with a measuring tape. The shadow boxes should be spaced equally from one another, as well as equidistant from the chair rail molding and the baseboard molding. In some homes the boxes are square and in others they are rectangular. Use the measurements from the wall to help you draw out a layout for the boxes.
Set a miter saw to a 45-degree angle. All pieces of the molding will be cut at a 45-degree angle. Place the molding on the miter saw table and cut it. Repeat with all pieces of the shadow box moldings.
Test-fit the cuts for the shadow boxes. If cut correctly, they should fit together like large picture frames.
Measure the wall and mark it with a pencil where the boxes will be installed. Set the cut pieces of molding along the marked wall. Use a pneumatic nail gun to nail them into place.
Cover the nail holes in the shadow box moldings with drywall mud. Let the mud dry completely and then touch up the paint. Caulk around the frames as needed. Spread a small bead of caulk around the frames and smooth it down with your finger. Let the caulk dry according to the manufacturer's directions.