Installing a mantel shelf is much easier if the front of the fireplace is made of brick and the bricklayers installed support masonry upon which a shelf can be placed. For example, a traditional red-brick fireplace front can have some of the bricks mounted so they point outward, leaving a 4 inch overhang that makes an excellent attachment point for wooden shelves. If the mantel is solid brick with no brick overhangs, then affix the shelf to the mantel by means of solid masonry screws. A drill with a masonry bit is essential to avoid damaging the drill and cracking the brick.
Solid wood mantel shelves come in a variety of shapes and styles. Be sure that the mantel shelf can support the weight put upon it. Delicate wooden shelves may not hold well if heavy ceramics or brass objects are placed on them. Make sure that a shelf being retrofitted to a wood mantel will support the mantel's structural stability--some mantels are merely front pieces that screw into the drywall around the fireplace, so screwing a shelf into the mantel may put undue stress on the screws holding the mantel to the wall. If possible, sink screws directly into studs, or verify that the mantel is solidly attached to the wall before installing the shelf.
Make sure that any wooden mantel or art materials are either fire-retardant or do not pose a risk of burning under normal fireplace heat. Some chimneys generate heat that can melt certain low-quality plastics or damage items like LCD-style TV sets. Use a screen with wooden frames when burning wood in a fireplace.