How do I cut a perfect square out of paper for origami folding? 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 &amp; 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?
This works better if at least two of the sides are parallel. It is required that one corner is at a right angle.

When I am trying to get perfect squares in paper I fold the paper in half diagonally. Not across the center exactly but more so that the two sides that make up one corner are brought together. Any paper that sticks out after that is removed.



In the above image the paper is fold from the bottom left into the upper right so that the top edge has no overlap. The grey is what would be waste.

Getting rid of that excess paper can be challenging as there is usually only a sliver at times. What I do is put a metal ruler down along the edge and use a craft knife. Start where the paper is thicker. Avoid moving the ruler even after the first cut is done. If you have to get rid of left over slivers or tufts you can usually take a few short passes without ruining the new edges.