We've all been there. You think it's a square. It looks like a square. You fold it into a bird base, and well, it's that tiny bit off. And that error multiplies as you go farther and farther down the line to the finished model, until all the compounding inaccuracies keep you from having neatly finished details and your model looks messy. Or you've used a guillotine cutter that looks like its edge guide would be accurate, but isn't.
How do you start with the perfect square? Are the recommendations for the types of cutters (guillotine, rotary, straight-edge & knife) to use? Is there a way of guaranteeing a perfectly proportioned square for folding? Especially for larger squares for complex models you may be cutting from rolls of paper, rather than sheets?