First of all this is just how I prefer to make tanks, but I think this would be a pretty good guideline for most players, some of these things I'll talk about are very basic, some aren't if you have any questions, please ask. In this tutorial I assume that everyone has access to XML and nudge, but if you don't substitute difficult parts (such as tracks) with structural panels. I uploaded some WIP version of my Cromwell whilst building it, I suggest that you check them out to get an idea of what I talk about in this post, Beginning and Completition

Step 0: Find something which you think that you can build, and look for things on whatever vehicle you want to replicate that you can't build. If you do find such a thing, either don't build it, or make a compromise. The sort of things I'm speaking of is the turret of the Chieftain.

Step 1: Find good pictures of it, both actual photos, and blueprints. Then, start building the hull, you want to decide how wide it is first. I usually just set the width to 5, and then select the height of the fuselage to make it look right.
For doing this you should select the place of the tank furthest forward which is: as high as the hull gets, as wide as the hull gets, and a rectangle. Example of pictures.

Step 2: From your rectangle, add the basic shapes of the hull, but make sure that you don't build it in such a way so that you can't remove any parts in the future, and also make sure to make it easy to change any dimensions, because you're pretty much guaranteed to have gotten it wrong at the first attempt.

Step 3: Make sure that you've gotten the proportions right, if you haven't this is the last point at which it is easy to fix it, and then start adding the biggest (roadwheels, sideskirts and tracks mostly) details to the front, side and rear of the tank. Check the "Airplane Properties" tab to make sure that your tank has a somewhat close to the original Length/Width.

Step 4: Add a rotator ontop of the hull, at the centre of where the turret will be placed. Then, build the turret ontop of it, and make sure that the center of the turret is on the center of the rotator, then nudge the turret and rotator to whrre they should be. Now you want to start adding details (to the turret, not the hull), but not the gun.

Step 5: Add the gun, you want to place a rotator to which you attach it on top of the turret, and the nudge rotator so that it can't be seen, and after that you nudge the gun into correct position.

Step 6: Add the rest of the details to the hull, and when you're done with that, add propulsion. I really, really recommend VTOL, but that's up to you. After adding engine(s) nozzles and inlets, nudge them into the hull so they can't be seen. Then, add small landing gear, I suggest placing them on the roadwheels, and, you guessed it, nudge them upwards to reduce visibility.