On top of the door-frame header add the mantel top (most ornamental door frames are not wider than five to six inches at the top). Add one or two pieces of vintage wood, make sure that you step them out on the front and the sides about one inch to create a layered look. Try to make the mantel top around 10 inches wide. Use wood glue and screws to secure the top to the finished door frame. Sand and caulk the seams.
Decide exactly where the top of your mantel will go. You'll probably want to place it higher than your current mantel to cover all holes left by old mantel. Measure the total height of your finished mantel; if necessary add wood blocks to the bottom of the fluted columns surrounds to add extra height. There will be one more board added to the top at the end so account for another 1-1/2 inches or so. Cut the additional blocks to size, make them a bit larger than the fluted sides to create a small revel on the front and sides. You may want to give them a beveled edge or sand over the top to give an eased, smooth edge.
Toe nail with small screws onto fluted sides for temporary support (it will be more secure when in place). Use wood glue where necessary and fill holes with wood filler. Use a caulking gun and paintable caulk to blend in all the seams.
To measure for the marble placement, put the finished mantel and surround in front of the fireplace in its exact location. The marble will go slightly underneath the mantel so there is no gap between the two. Measure from just inside the edge of the firebox (add an 1/8" to over hang) to just underneath the mantel and surround. When you have all your measurements, carefully set aside the mantel and surround.
Use a wet saw to cut the marble pieces to size. Spread heavy-duty stone adhesive to the back of marble pieces that will go on the sides of the firebox. Set in place (1/8 of an inch over the edge of the firebox sides) and level. Place boards and weights up against the marble pieces to hold them in place while the adhesive dries. Allow them to dry for at least 12 hours before placing the top piece of marble. Then place the top piece of marble, all the way to the ends of the two pieces on the surrounds. Push in place with wood structure and weights let it dry for another 12 hours.
Put the new mantel in place, use drywall screws to fasten it into the wall studs. Place the mantel against this frame and wood glue and toe nail screw together. If the mantel is away from the wall a little, add more wood trim to cover the gaps. Toe-nail the top of the horizontal boards into the wall studs. Caulk and sand, let that dry, then paint entire mantle piece. Let it dry and then add a second coat of paint if necessary.
Joanne Palmisano is the author of Salvage Secrets (W.W. Norton, September 2011). Visit her blog, also called Salvage Secrets.