Sketch the Design
Sketch the design for the fireplace mantel and assemble the required tools and materials. This project uses stock pine and ready-made wooden details - including plinth blocks, and molding - all purchased from the home improvement store. Consequently, most of the work involves making simple crosscuts, rips, and miters and assembling the components with a pneumatic nail/brad gun.

Remove Existing Molding
Remove the existing molding, by sliding a large putty knife behind the molding to protect the wall while you lever it away from the wall with a small pry bar.

Make Crosscuts for the Face Panels
Make the crosscuts for the face panels of the fireplace surrounds from standard dimension pine lumber with the miter saw or chop saw after taking measurements. The basic structure consists of two vertical surrounds (columns) and one horizontal surround, each built with a face panel and two narrower side panels that will butt against the wall.

Rip the Pine
Rip the pine with a table saw and fence to form narrower side panels of the same length as the face panels. On each of the three surrounds, the side panel that rests on top of the tile will be narrower than the one that abuts the wall - narrower by the thickness of the tile.

Assemble and Glue the Top Surround
Assemble the top surround by applying glue to one edge of each side panel and butting them against the underside of the face panel. The edges of the joints should be flush.

Secure the Joints
Secure the joints with a pneumatic nail gun. Repeat steps 4 through 6 for the two columns/surrounds.

Align the Top Surround Above the Fireplace
Hold the top surround in position above the fireplace. The shorter panel goes on top of the tile. To help align it, you should measure and draw a line with a bar level. You should also have marked wall studs around the tile work after locating them with an electronic stud finder.

Use Bar level to ensure Accuracy
Confirm once more that the top surround is level by laying the bar level on top and ensuring the bubble centers in the glass.

Nail the Top Surround
Toenail the top surround to the wall/studs behind it with the pneumatic nail gun.

Glue Surround to Tile
Put multiple dollops of construction adhesive along the interior edge of the first surround/column where it will butt against the tile. We cannot nail through the tile, so we will use adhesive to hold it firmly against the tile.

Attach Column
Attach the column, nailing through the surround and into the wall with the nail gun. Repeat steps 10 and 11 for the second surround/column.

Glue and Nail Decorative Molding
Glue and nail together a piece of one-by-ten pine and strips of decorative molding along the front and sides to form the mantelpiece. You should miter the corners and set the decorative molding back from the edge of the one-by-ten approximately 1/8 inch to add some extra detail to the profile.

Add Glue to Upper Edge of Top Surround
Add dots of construction adhesive to the upper edge of the top surround to hold the back of the mantelpiece.

Nail Mantelpiece to the Top of the Surround
Set the mantelpiece in place and drive nails through the mantel into the top of the surround with the nail gun.

Nail Plinth Blocks
Nail plinth blocks at the top and bottom of both side surrounds/columns.

Nail Narrower Plinth Blocks
Nail narrower plinth blocks at the sides, lapping over the edges of the plinth block that you just added to the fronts of the columns.

Cut Decorative Molding
Cut decorative molding for the face of the surround, using a miter saw.

Attach Smaller Molding
Attach the smaller molding with a pneumatic brad gun.

Miter, Position and Nail Decorative Molding
Miter, position and nail decorative molding to make the transition from the tops and bottoms of the plinth blocks to the face of the columns. Use the brad gun to attach it.

Apply Spackling Compound
Apply interior-grade spackling compound to fill all the nail holes.

Paint the Surround
Paint the surround.