This tank was designed by my brother for a building competition we entered. We made the top 10 out of 170 teams. We created an alien invasion themed map and we needed some military vehicles to protect our town. Instructions on how to build some of those vehicles, like one of the UFO’s and a fighter jet, can also be found on this site.
Note that the original design was made with slabs instead of stairs, as minecraft didn’t have the upside down stairs yet when this tank was designed. I later altered the tank to make it look better, though both versions can be seen and downloaded on this page.
The full instructions are pretty much all covered in the image below, you can click on it for a large version. I’ve tried to cover everything as much as possible, without using too many steps. If you feel the instructions aren’t clear enough, feel free to ask for more details on the forums.