Here are the columns I started with...not sure what they were from. Don't they just scream "fireplace" to you? See those 2 unfinished pieces to the left? They're screaming "quit procrastinating and finish me!!"
After de-constructing the columns to learn that 2 sides from each had about a one inch lip on the edges, I set about removing that lip so the boards would lay flush side-by-side. Sorry for the blurry picture.
Next, I layed the boards out to determine my width and hope like heck I'd have enough for the sides. Did I have a plan sketched out? Did I measure? Nope...100% eyeballing at this point!
Next, I flipped the face boards over and secured them all together with some glue and horizontal planks. So far, so good.
Once I had the form all together, I trimmed the top & bottom and added some thick plywood for a back. Then (this time I measured) I cut the opening.
Dry fit the mantel, eyeballed some more...lookin' pretty good. Attached the mantel and a beefy corral board for the bottom. Darkened with a secret stain/paint combo for some contrast...
Next I embellished the front with a metal fleur de lis to "French" her up!