ASSEMBLE THE BOXES. Start by taping assembling and taping your boxes together to create your sides and hearth. Then, using either packing tape or spray adhesive, cover the boxes in newspaper or wrapping paper. This will hide the seams of the boxes.
MAKE THE BRICKS. Using scissors or a box cutter, cut out your cardboard "bricks". I eyeballed the size, but they ended up being around 3" x 8". Then tape them on in a staggered pattern around the outside.
COVER THE BRICKS. Using your hands (it's easier than a putty knife), spread the joint compound all over, making sure to cover all the cardboard bricks and paper.
MAKE THE MANTEL. Tape your pizza boxes together (it depends on your fireplace size, but we used 3), then wrap them in the wood contact paper and them tape to the top of your fireplace.
MAKE THE INSIDE BLACK. With your packing tape, attach the black poster board (or a sheet of cardboard) to the back of the fireplace. Then cover the inside with black wrapping paper or paint.
Place your fireplace where you want, decorate, and enjoy! You can add fairy lights, faux (or real!) wood logs, etc. just to make it even more realistic and fun!