I attached the frieze board to the inside edge of the pilasters with screws through slotted holes to allow for wood movement. The vertical members are attached to the inside edge of each pilaster with screws through the backside.  

    The next step was making the three architectural moldings I planned to use from cherry. I used architectural molding router bits from Eagle America. Initially, I was concerned about burn marks on the cherry but by turning the router's speed control all the way down and making multiple passes, I was able to get clean crisp burn-free moldings.

    This is a close-up view of the moldings with the mantle shelf resting on top and the cherry bead molding dry-fitted in place. I was able to take the mantle shelf over to the house and scribe it before bringing it back to my shop to remove the wood indicated by my scribe lines. I couldn't  attach the cherry bead molding or mantle shelf until installation.
     The bead molding will go against the slate tile that surrounds the outside of the firebox and cover the gap between the slate and the edge of the mantle. A mounting cleat must be attached to the wall just below the mantle shelf before it can be attached.