Lay your mantle face down on the floor in front of the fireplace on top of an old sheet or tarp.
Locate the studs around the fireplace using a stud finder. The mantle will be nailed into the studs to achieve a secure fit. Mark the location of the studs with a pencil.
Apply a 1-inch wide bead of liquid nail adhesive to the entire backside of the mantle and then stand it upright. Put it up against the fireplace in the desired location and check with a level to make sure it is placed evenly.
Predrill holes using the cordless drill, and at a 45-degree angle through the mantle into the studs. Screw wood screws in through the legs of the mantle and the underside of the shelf. If you are attaching the mantle to stone, use a masonry drill bit with masonry nails instead.
Fill any divots made by the screws with the wood putty, and smooth then out with your finger or a putty knife. Allow the putty to dry, and then smooth it further with a piece of fine sandpaper. Paint or stain the mantle to your liking.