There are a couple of ways to go. One would be to purchase either some complimenting wood or metal brackets or corbels and securely mount them to the framing in the wall, then the mantel can be attached to these brackets. BTW, a cabinet supply store will usually carry some pre-made wood corbels of various sizes and species. Another way would be to construct upright supports that could mount to the wall on either side of the fireplace opening and attach the mantel tho these. Be sure and check the code for mounting clearances to the fireplace opening (since the mantel is combustible). The further the mantel extends from the wall the more clearance will be required.
