I used 15"x15"x48" boxes for the sides supports.
I used white latex house paint for the "wood", and "Boston Brick" (Behr brand) for the bricks. After the spray-on grey mortar was dry, I sponged on the bricks. I mixed some white into the brick color to lighten it, and sponged a second coat to give the bricks a more realistic look.
I used a third 15"x15"x48" box (opened up) to make the front, top, and part of the back section.
For the mantle, I used 3/4" white pine and trimmed it with some 1"x3". Final measurement is 18"x70". I secured it to the top of the boxes with some Liquid Nails.
I kind of winged the grate for the logs. I hot-glued two layers of cardboard together to make the side rails, then cut strips to use as cross supports.