WVRR, I d/loaded the burgandy coloured semi from the the site Sir-Prize posted in your other thread. Here are some observations......
The trailer was larger in scale than the cab! Trailer was approx O and the cab was HO.
The d/loads are *.jpg files so I suggest using PSP Version 7 (or 8) to remove jpg artifact and save the result as bmp.
I guessed that the width of such a vehicle is approx 8ft in real life ... (Someone correct me please if I'm wrong). Copy a portion of the cab or trailer rear precisely at its width and paste "as a new image". Change the "ruler" to mm and the new image will indicate the width of the d/load. Go back to the overal drawing and "resize" by "percentage" once you've established the ratio of the Drawing to the size (scale) you require. My N effort turned out to be 9ft wide! but I was quessing at the % reductions to make.
You're correct about the fold tabs around a cab, man, they sure are tiny! but time spent studying the layout of the drawing should reveal which are tabs.
After cutting out the pieces but before folding, run a black (or appropriate colour) felt pen along all the edges which will be visible on the finished model. This avoids "white" edges of the paper/card spoiling the overall effect.
I was in a hurry and printed to photocopy paper which is a bit too flimsey for N scale. I will try 90gram paper next. The colour of the trailer depicted below is incorrect (cheap printer)!
Trailers will probably look better and be easier to make if the black chassis is made from cardstock and will give a degree of rigidity to the model.
My eyesight is fair but not good enough to make the tyre treads as proposed .... I would hope to find craft beads approximating the size of wheels needed. I would also make a subframe for the trailer wheels as they simply don't look right as part of the trailer sides.
I used Google to search for "Paper Models" and there are dozens of sites with advice on constructing card and paper models though I didn't open any of these sites.
I haven't made the cab yet.
We live and learn!
Errol