A short instructable story of how we created our first cardboard fireplace for the office. We actually used two sets of boxes for the task because we messed up the first time. It's a moderately easy project which requires skills in crafting. For the fireplace you will need:

Three boxes - to form an arch. The boxes should preferably be bigger in height and smaller in width. The first set we used, were from some sports equipment and the second set - from three propane bottles. We got them for free from a local home improvement store.
Paint - the colour depends on your interior. We used white interior wall paint. We had that lying around from a recent home renovation. You won't need a big quantity, don't bother getting those big 5 litre (over a gallon) buckets of paint.
Wallpaper - preferably brick decorative wallpaper, so it can look realistic.
PVA glue (or whatever other glue you can get your hands on) - to hold everything together.

For the fire pit, we used:

More cardboard (who'd knew)
Christmas lights - make sure they don't get too hot. We don't want to set our fireplace on actual fire.
Some duct tape

Step 1: The Boxes

As said, we got our from a home improvement store in London - Wickes. Some of the employees there are really decent people and give out boxes for free if you're kind enough to ask. Chances are, Home Depot guys are the same.

First, we applied the paint as shown. Do that first because it gives you some room for mistakes.

After that, we applied the brick wallpaper. We got our from Wickes again, but it's not on sale anymore. You can get this one, instead. Home Depot, on the other hand have a pretty huge selection of brick wallpapers. You can browse them here and get whatever one you want.

Step 2: The Finished Fireplace

The end result can be seen on the first photo. Pretty good but it lacked something. One of our colleagues brought a kitchen top left from a renovation. Happily, the structure was sturdy enough to hold it. Now for the "fire".

Step 3: The Fire

For the fire stand, we've cut 4 rectangle shaped cardboard pieces each 45 centimetres (18 inches) long and 10 centimetres high. And two shaped like the letter H, which were at least 60 centimetres (24 inches) tall. We painted everything black with spray.

For the actual fire, we took two cardboard pieces and folded them together. On the top of the top one, we used a paper knife to make it look like burned wood. After that, we painted them black with a brush. Painting it after will cover the edges we cut earlier and make it look more realistic.

After that, we placed the Christmas lights in there and held them together with some mighty duct tape. Make sure your lights are not heating too much. Otherwise, you might get too realistic with your project.