Check local building codes for minimum setbacks from the firebox allowable for wood trim. Measure both the firebox and the adjoining wall space that you want to dedicate to a surround.
Remove existing trim with a wide putty knife and a small pry bar. Position the putty knife between the pry bar and the wall to prevent gouging. Consider replacing the tile or brick to update the hearth and the firebox.
Components of the fireplace surround are a horizontal strip of wood across the top of the firebox and butting up to the inside of each column, decorative columns on either side, and a mantel, which will rest on the horizontal piece and the columns. Cut the horizontal piece to the desired length and depth. Locate wall studs with your stud finder and mark their position, as well as the position of the horizontal piece. The top edge of the horizontal piece should be at the same height as the top of the decorative columns. Make certain that the horizontal piece is level and nail it on the wall, using a nail gun, or drill holes and use a hammer and nails.
Position the decorative columns, gluing the inner edges that touch the tile. Make certain that they are resting on the floor and are level and plumb. Nail into place, using a nail gun or a drill, hammer and nails.
Position the mantel over the horizontal piece and the columns. Check to make certain it's evenly placed over the three pieces and mark the center spot on both the wall and the mantel. Remove the mantel and put carpenter's glue on the upper surfaces of the horizontal piece and the columns. Position the mantel again, check for level and secure with nails.
Spackle nail holes and other imperfections prior to painting. Let dry and sand. Apply paint or stain.