Step 7: The total beauty of corrugated cardboard
Corrugated cardboard isn't a finely honed engineering material: Because corrugated board varies so much in its thickness and composition, and can be squashed and bent to a certain extent, the plans do not need to be millimetre precise - but make them as close as possible. In particular, the holes in the struts should all be aligned with each other, and at the very least, each left-right pair should be the same.
If you have printed the plans, just hold them onto your cardboard panel and use something sharp (a nail or a metal skewer - I use a scribe which is ideal) to punch lightly through at each corner on the part to cut out, then remove the plan and just join the dots using a ruler.
Corrugated cardboard is a bit like balsa in that it can "catch" on a blunt blade and end up tearing, and making a ragged edge. In corrugated board board this tends to be a bigger problem when cutting along the grain. Use a good long sharp blade so you can keep the angle of the cut fairly low, and use many shallow cuts instead of one deep cut.
The base connectors and the gondola have folds in them, and that's potentially a great source of variance. To fold the board, use a straight edge and something blunt to compress the board as much as possible without cutting through it. Tearing the top layer is no problem, but be careful deeper. For bends tighter than 90 degrees, use something flat and blunt to compress the corrugations for 10mm or so either side of the inside of the fold too, so that there is less of a tearing force on the corner itself. Just keep an eye on your folds and you'll see where needs extra attention.
Laminating / Gluing:
The struts in this project are made of a double-thickness of double-layer board, just glued together. I used PVA glue. This actually adds a lot more strength and tear-resistance than just the two layers of cardboard alone, but it can be messy. You could probably get away without gluing the struts on this project, but I'm playing it safe.
When laminating, make sure any curvature in the board is opposed by the other board, so hopefully it'll dry flat. Put a generous layer of glue on one board and spread it around so there are no gaps, then put on the other board and load it up with books or some other flat stuff. Use clingfilm or scrap paper to stop the glue squeezing out the sides from going where it shouldn't. Leave overnight. The glue continues to harden after that, so it will become tougher to file and cut after that.
All the holes for the bamboo skewers should be punched through initially with a scribe or a nail or something, then gradually reamed out a little until a bamboo skewer can turn easily in it, but not so much that the skewer is loose. I found I had a thin needle file that I could use to widen the hole. It was a pain to do so many, but did the job.
Personally, I think this method of making holes is preferable to actually cutting holes (where material is removed). This way of doing it folds the material back into the hole and lines it nicely, making for a larger bearing surface for the skewers.
When working with this corrugated cardboard, you should have soft hands, like when catching a ball. It's quite easy to compress the corrugations when trying to be forceful with it, and once it's been crushed or bent, it loses much of it's strength, so be careful.
The glue layer, in particular, is surprisingly tough and I ended up compressing the bottom layer of the laminated boards a bit when trying to cut it.