I'll try to help you with whatever knowledge I have, though I beleive there are others out there who have more tips and tricks than I can offer, but I will certainly try my best.
The most important thing you must know is how to Layer the Cardboard, so the width is in proportion. And several parts are wider that other parts, you must take that into account. Look at the weapon of choice in Halo 3 to see which parts are wider or smaller, that should give you an idea. I will leave the printing of Blueprints to another member to explain, I can't help you there.
Another thing you must take into account are hollow parts (Magazine Feed, barrel hole, etc). Experiment with the layers, cut out the inner layers so they fit a magazine. Then cover the inner layers with one outer layer on each side, making the inside hollow. And you have to take into account angles, find a way to make the angles perfect.
I hope this helps.