My supplies consist of a piece of plywood, cut in half. A 2x6 scrap wood from our contruction demolition and a wedge to support the middle. Some trim. Paints, including chalkboard paint left over from this project. And a screwgun and screws which I forgot to add.
I painted the plywood with the chalkboard paint for the fireplace hole, the trim, and the rest of the plywood with some kind of stone or tile pattern. You can paint it whatever you'd like.
I screwed the 2x6 directly to the plywood, then the plywood into the wall. Don't forget to add the wedge underneath the mantle for added support.
Don't mind our messy construction you can clearly see to the left of this wall. If you do this project, or something like it, I'd love to see it!