Step-1

What will it do? I will be covering a main battle tank style vehicle, however, other vehicles are necessary to facilitate proper combat.
The most important of these vehicles are: CEVs (combat engineerimg vehicles) and ARVs (armored recovery vehicles).
Both can be built out of the hull of a main battle tank, infact, this is recommended for logistical reasons.

Step-2

Build a hull.
The fron should be angled sinfigantly, but not in a way that creates a shot trap. A shot trap is where a round will be forced off an angled part of the tank and directly into a vital component or relatively weak armor. It is best to keep it simple at first......and make it look realistic.
Also, make room for both the turret and driver's compartment.....with an hatch. Also, create some type of reallistically shaped engine compartment.
The easiest way to create hull for most tanks is a 25'x4'x8-10'.
It can either be two 1 block thick plates put together or a single two block tall fuselage. It is also possible to make the tracks' fenders/guards from the top of the hull.
Example