Here’s the first of an ongoing series of diagramming tips, assuming you’re using a computer to produce them. They will all be specific to Adobe Illustrator, but the principles should apply to any drawing package.  In the UK & Europe, we do a lot of folding from A4. Rather than creating a rectangle of the correct dimensions by typing them in, here’s a method that uses the properties of A rectangles to find the appropriate length. The property in question being that the length of an A rectangle is the same as the diagonal of the largest enclosed square.

1) Create a square and duplicate it
2) Rotate the duplicate 45 degrees (grab a corner, hold shift to constrain angles and drag to rotate)
3) Expand the width of the original square so it touches the corner of the rotated square
4) Delete the rotated square.

Once you have the shape, you can add it to a library for future reference. In Illustrator, these are called “symbols”.