Buy or cut a quartersawn piece of oak to the dimensions you've determined for your mantel. Look for a long-grain pattern that extends the length of the mantel, with rays and wavy grain patterns emanating from the center of the grain.
Stain this wood with a medium or dark stain. Add varnish to enhance the grain.
Find the center point of the wall over the firebox, and determine the height of the mantel. Check with local authorities on their recommended clearances, but plan on the wood being at least 8 inches from the firebox. Mark the location on the wall.
Drill holes in the backside of the mantel to accommodate the mounting hardware. Check the weight of the mantel piece and buy the correct mounting hardware to hold the weight.
Attach mounting hardware to the wall, using a stud finder to make certain it connects with a wall stud.
Apply carpenter's glue to the pre-drilled holes in the back side of the mantel, and slide the mantel over the mounting hardware. Check for level and adjust as necessary.
Touch up the stain as needed.