Hello there, my name is Mitch. A model which I define in minecraft is an object that wouldn't be constituted as a building or a sprite. Buildings are what would be thought of as arcitechtually sound. Walls, roof, etc. Churches, homes, Reichstags and the such go in this catagory. A model would be things like statues, vechicles or objects; such as trucks, guns, guitars, pill bottles, whatever. I have made various models, and have found what I believe to be the easiest way do to this accurately. I have done various firearms, such as the MAC-10, AK-47, G36C and .44 Magnum. I have also constructed a 'Boomer' from Left 4 Dead, and a ballista. All of these done using the following method.
It starts in 5 steps, and all of them require your utmost skill and attention.

1. Outline
This is harder then it sounds. Essentially, you need to get a picture of what you're doing as a guide, hopefully at a good angle. Longways is easiest, so for a car or firearm you'd do it from the side. For people, front is best. What you need to do is just make an outline of the model, all along one axis. It will have to be flat, as such:
http://tinypic.com/r/2cgcxh5/5

2. Body
This is basically done by adding blocks onto the outline going in from the flat axis. You need to do the basic shape of the model, building IN from the outline. This is easy to do based on how many blocks in you go. Only do one side for the moment. With people, you do the shape of the body. Clothes and face come later.
http://tinypic.com/r/2r3xmwp/5
Now, with guns, cars or anything with symettry down the flat outline all you need to do is copy the side you did onto the flat side. For people, however, whose backs are NOT identical to their front, you need to do something differant. Again, get a good angle pic for this.
http://tinypic.com/r/10qdn39/5

3. Body addition and detail
At this point your model is looking crummy. Guessing your way through it can't be good. So what you do here is fix up your body. Whatever looks bad should be changed. Take a few steps back. Does it look good? Bad. Does it look great? Good! The body is what defines your build, you NEED it to be as accurate and as good looking as you can get it. Now what you need to do is add additional detail TO your body. Clothes and face for people, sights and whatnot for guns, strings for guitars, whatever. It's also here that you should mess around with the colour of your body if need be. Try to have your body's colour as varies as possible unless it doesn't make sense. For granite statues, everything is done in grey rock, but you can do some segments in light grey and the like.
http://tinypic.com/r/v8q2py/5

4. Step 4.
This is what you put ON your body. Now, for some this and the next step is not required, but it does give some types of model an edge. This is the fidgety little accessories that make something from good to great. Add whatever you didn't before to your build now.
This is mandatory for vechicles and some firearms, but for statues, however, it is not needed.

5. Step 4, part 2.
Almost there. Now take a few steps back and look at it again. What looks like overkill? Try to evenly distribute accessories. Fix up any faulty looking Step 4 details.
Be sure to add your name.

Note, that not all models can be done easily. It is a complicated task, especially task 2. I myself have failed to finish a model due to its' difficulty (though to be fair it was a minigun). Sometimes things just don't seem to work, but keep at it. With sufficiant pratice any model can be completed.
For my previous models(not all of them) which have been done with this method, look below:
Chainsaw http://tinypic.com/r/4h8ms3/5
Ship: http://tinypic.com/r/mrzthf/5
Ballista: http://tinypic.com/r/209qahl/5
G36C http://tinypic.com/r/4qbts/5
Globe: http://tinypic.com/r/346qeeg/5
Revolver: http://tinypic.com/r/2v93ujo/5
Uzi: http://tinypic.com/r/dr9g7s/5
Map of world: http://tinypic.com/r/eskaaq/5

Thanks Twangykid for the help on the construction of my Christ the Redeemer statue, which was used on this tut.

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