Start by ripping down and cutting the kirei board to the desired dimensions: Top and bottom pieces at 70-inches long by 9-inches, two matching sides at 4 by 9-inches and the front at 70 by 40inches. The final dimensions of the mantel will be 70 by 4 by 9-inches deep.
Make 45-degree miter cuts along all four edges of the front piece. Then cut 45-degree angles along one side of the kirei and at each end for the top and bottom pieces. The side that will not have a miter cut is the side that will attach to the wall. Finally, miter the two end pieces on three sides, leaving the side facing the wall straight.
Lay out the pieces with the mitered sides facing down. Stagger them with the front piece in the center and the top and bottom pieces on the side with the flat edges facing out (Image 1). Apply painter's tape along the seams where each piece meets (Image 2). This will assure the pieces do not shift. When using tape, make sure to use a type that is compatible with the surface, otherwise it may damage the material.
Turn the piece over so the mitered sides are facing up. Generously apply wood glue into the crevasses of each miter cut.
Add glue to the mitered cuts of the sidepieces and then apply at each end. Remember to place the flat end of the piece towards the back. Add tape over the seams, trying to make them as nice and square as possible. Then go back and place tape across the end pieces holding everything nice and tight.
Once the mantel is all taped up, let it dry overnight. Once dry, remove the tape and apply a water-based stain to bring out the beauty of the kirei.
Cut a cleat that will fit into the back of the mantel. To figure out the measurement for the cleat, simply subtract the thickness of the wood from the measurement of the mantel, in this case 67-3/4 by 1-3/4 inches. Taper the ends of the cleat so that the mantel will slide on more smoothly.
Slide the mantel onto the cleat and secure it with 3-inch screws on the top and bottom.