Using a bead of carpenter's glue on the top of the hanger board, place the mantel on the hanger board.
To attach the legs of the mantel, use another bead of glue on the back side or toenail in with finishing nails.
What if we want to cover all the brick for a complete transformation?
Code requires a clearance to combustibles so you can't install sheetrock directly onto the brick face wall. Mike suggests covering the brick with noncombustible cement board (the adhesive used depends on the surface you're covering) . Cement board can be mudded, taped & painted to look like sheetrock, or can serve as a flat surface to install tile, marble or other facing materials.
Ceramic tile was installed here in the interest of brevity for the demonstration, then the mantel was reinstalled to overlap the tile.
Ceramic tile can be installed directly onto the brick wall since both are noncombustible, so long as the correct adhesive is used and the brick wall is very flat.
Where the tile is shown in the photo, you could use slate, marble, granite, etc. to completely change the look. To hide all the brick, simply cover it all with cement board then mud, tape, finish and paint it.
Check codes to determine the required clearance of noncombustible facings between the opening of the fireplace and combustible mantels or trim. 8" at the sides and 10" at the top is common. If you have a stove or vent-free gas logs installed in the fireplace then clearances from the appliance itself may also come into play.
When using marble as a facing material, do not use carpenters glue to adhere it; marble is porous and the glue can leech into the marble and stain it. Ask your marble supplier what type of adhesive is recommended for the type and color of marble you're using.