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.
-Mech Tanks Mod