Steps:1. Measure the dimensions of your existing mantel and cut down the 8-foot plywood into two long strips that will fit around the mantel when assembled. Be sure you allow an extra 1-1/2" to the top and front length to accommodate the 3/4" sides. Cut down a third piece the same length that will have a shorter width to tuck beneath the mantel.
2. On one of the large pieces of plywood, create screw holes with a drill and pocket jig about every eight inches (figure A).
3. Glue the first two pieces together at a right angle. The pocket holes on the first piece should face inside. Dont add too much glue because this type of wood doesnt absorb it too well.
4. Secure the plywood together through the pocket holes with hex head screws. Add clamps as you work your way down to help the glue dry.