I've made dozens of posterboard models of my own
designs. I think I've gotten it down pretty well now...
The first thing you need to do is get printed
out copies of the expanded panels and bulkheads in the scale
you want to build. For blueprints, I'd suggest you run down
to Kinko's and get copies made for a few dollars. You could
also trace the shapes off the plans if you are truly frugal.
I do all my boat shaping using Carlson's Hulls
program. It gives the panel expansions in .DXF format which
I import into a CAD program and print them out.
Then run down to the hobby store and buy a can
of aerosol spray adhesive. Spray the back side of your copies
or tracings and glue them to dull side of the posterboard.
Cut all the bits out with scissors.
I then temporarily tape the model together with
little tabs of tape on the glossy outer surface of the poster
board. The blue 3M masking tape works really well for this because
it will release easily. After it is all taped up, the all the
joints get glued from the inside with a hot glue gun.
Remove the tape and spray paint. Voila! You've
got a great looking model.