We used an old ornamental door frame, some reclaimed molding and a few pieces a salvaged marble to create a new fireplace mantel and surround.
Use a hammer and a crowbar to carefully remove the mantel and existing surround. Our fireplace did not have a surround so we are going to adhere our new marble surround directly to the brick.
We used two pieces of an ornamental door frame which included the header and fluted-column sides. Measure and cut your lumber to the mantel size for the mantel's vertical pieces. The cut the door frame header to make the horizontal piece. Depending on the size of your door frame, you may have to use two pieces. If so, use a miter cut to help conceal the center cut.
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 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 to add extra height. There will be one more board added to the top 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.
We used three pieces of marble for our new surround. 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. Set in place (1/8" over the edge of the firebox sides) and level. Place boards and weights up against the marble to hold it in place while the adhesive dries. Allow at least 12 hours drying time for the sides before adhering the top piece of marble. Place the top piece of marble, flush with the ends of the two vertical pieces. Push in place with wood structure and weights let it dry for another 12 hours.
Stand the new mantel in place, use drywall screws to fasten it into the wall studs or masonry screws if fastening to brick. If the mantel sits away from the wall, add more wood trim to cover the gaps. Toe-nail screw the top of the horizontal boards into the wall studs.