It would probably take quite a while to build a machine of that size, though to be fair the Fire Nation would probably be able to take advantage of occupied Earth Kingdom labor along with their expert engineers.
Not really sure what the second question entails. Start with plans, build superstructure, wheld on coverings, and add details, I suppose.
It would probably be hidden where a lot of secret things in our own world have been hidden: underground or in a mountain. They could have probably repurposed an old coal mine or something to fit the task.
Considering Long Feng's taboo on discussing the war at the time, there may not have been all that much recon going on. Besides, with walls like Ba Sing Se's, they probably thought themselves untouchable, and so didn't bother with much intelligence gathering.
I know the generic 'how to build stuff'. I mean, how would someone go about building it? Where would you find the materials? And the people controling it, what exactly were they doing? The line of operators were just pressing buttons and turning dials.
For the materials, I would imagine the vast resources of the mostly conquered Earth Kingdom would be more than enough to supply the steel needed to construct something like that.
As for the controls... I have no idea. Its a piece of pure steampunk, I doubt much thought went into how the thing is steered or propelled. The logic there almost always goes gears/steam engine -> technobabble -> something happens.
it probably works like tunel bores do today they set them up to head for the wall and let her rip with the only controls being "head forward" "head forward at incredible speed" and "stop and alow yourself to be moved"
1. Idk how long it would take, but they definately exist: that's how they dig those massive tunnels underground for transportation and stuff. http://www.youtube.com/watch?v=Xj8-0w7s6qw Not quite as big as in avatar, but you get the idea.
2. Hard work lol.
3. Shhhh! don't share that secret...
4. Yeah, it's interesting/stupid how nobody did anything about it sooner: it had to have been built in the earth kingdom cuz they wouldn't be able to get it accross the ocean, and it was moving really stupidly slowly. I bet the general was just confident enough that his men could take care of it when ever they wanted.
in reality they should have been able to stop it. it wouldn't have been that hard.
1, have a few dozen soldiers line up create a huge trench
2. repeate the processes several yards away from the first trench(going away from the wall)
3. repeate this processes several times
4. once the drill passes all but the last one the soldiers drop the trench the weight of the drill will tip it foward, while the several dozen soldiers use earth bending to push the backend up in air then a dozen more soldiers smacking it from the side so it falls over,