Position the mantel where you want it and make a mark on the wall over the middle of the mantle. If you are mounting the mantel over a fireplace, comply with any local fire codes regarding the distance above the fireplace that the mantel must be.
Place a level just below the mark you drew. The distance between the level and the mark should be equal to half the thickness of the mantel. Create a level line that is as long as the mantel is.
Use a stud finder to find and mark the middle of the studs. Make small marks perpendicular to the level line you just made.
Drill pilot holes using the 1/2-inch bit for the lag screws on the marks you just made. If your mantle is at least 6 inches thick, drill a hole 1 1/2 inches above the mark and a hold 1 1/2 inches below it. If your mantle is less the 6 inches thick, drill one hole directly on the mark. Repeat this process for every stud.
Drill lag screws into each of the pilot holes. Drill the screws about four inches deep so they go completely through the studs.
Mark the remaining ends of the lag screws at a distance from the wall equal to two-thirds of the width of the mantel. Cut the screws with a hack saw on those marks.
Hold the mantel in place against the screws as if you were about to install it. Have a helper mark the position of the screws on the side of the mantel.
Place the mantel on the floor and drill 3/4-inch holes where the marks are. Ideally you will use a drill press, but if you don’t have one, make the holes as straight as possible.
Place the mantel on the screws, to test the fit. You don’t have to get it all of the way on, just get it on enough to ensure the holes are the right size and in the right place.
Apply construction adhesive to the screws and to the wall where the mantel will rest against. Slide the mantel over the screws and press it back into place.