Determine the height of the mantle on the brick fireplace. Most mantels are placed about 60" above the floor. Mark the grout line that is closest to the desired height. Place marks on the grout line every 16". Make sure those marks are level, centered and extend far enough to cover the mantel’s length.
Mark on the rear of the mantel where the lag bolts will hit. Make sure to measure and mark accurately so that the mantle holes will align perfectly with the lag bolts. Drill 1/2" holes across the back of the mantle to accept the lag bolts.