I was making railroad engineer hats and wanted to get the hexagon for the top. I cut a square 12 inches on a side. That was too small so I tried 14 inches. That was too big, so 13 should be just right.
I cut two of the squares, laid one down on the cutting board, laid the other one on top "on point", then cut off the edges sticking out on the bottom one. Worked perfectly. All the sides are the same length.
Now I just had to put in the pleats on each point and sew that onto the headband. I had to make a visor, too. Not so hard.