These can be easy to make. The trick is to get the project to look like a destroyed building rather than a collection of scraps! ;-)
Make your basic shape of the building by gluing scraps of cardboard (or foamcore) to a base for portions of the wall. You don't have to have four walls, three walls or a corner made from two walls are good options too.
Once you have the silhouette of the building start adding rubble, fallen walls, and other details to the building. The more details, the more realistic. Good ideas are: gravel, kitty litter, scraps of cardboard, foamcore or plastic, broken bits, etc.
You can also add detail by adding blast marks and holes to the walls and base.
When you are finished detailing it, spray paint black and then dry-brush subdued colors such as gray, brown, and brick red.
(The miniature is by GHQ)
This was one of my first experiments with building a city block. I used styrofoam blocks wrapped with tape and detailed with card windows, sand, and kitty litter. The big collapsed wall was made by melting the exposed strofoam with black spray paint. Then I detailed it with kitty litter and drybrushed with gray paint.