Place a drop cloth on the floor around the hearth to catch falling soot and debris. Locate and loosen the brackets that secure the old fireplace doors. Remove the fasteners and lift the old fireplace door unit out. If replacing the grate, remove that now too.
Mantles are usually 60 inches off the floor. With a tape measure, mark a line 60" above the floor. If there is a raised hearth, measure that separately to ensure an accurate distance. Measure the width of the fireplace to locate and mark the center point.
Position the 2x4 mantel brace on the line at the center point. Use a level to make sure the brace is perfectly horizontal. Mark and predrill holes to line up with the mortar joints. Attach the brace to the wall using 10-inch masonry screws.
With 45-degree miter cuts at all corners, place and attach the crown molding mantel to the brace and breastplate with 1-inch finish nails (Image 1).
Cut a bottom cap to fit tight to the bottom of the crown molding to give the mantle a solid appearance. Secure with 1-inch finish nails (Image 2).
Add any additional decorative trim to the fireplace mantel. Putty all nail holes, allow to dry and sand smooth. Prime and paint the fireplace mantle.