Mark horizontal and vertical lines with the level above and on the sides of the fireplace where the mantel will be located. Lean the mantel against the wall and check for proper fit and alignment. Mark the outline of the mantel on the wall.
Locate wall studs inside the marked mantel area. Drive a nail through the Sheetrock to locate the studs if necessary. Nail holes behind the mantel area are harmless and will not be visible after you install the mantel.
Drill holes in the mantel at the stud locations to use for wood screws or nail locations. You can angle the screws and nails if necessary to hide them. Countersink the head below the surface to leave room for wood putty or filler and paint. Secure the mantel at several locations on the horizontal and vertical pieces.