Originally Posted by
MikeM
Take a pencil and draw two lines connecting opposite corners - they will form an "X" intersecting in the middle. Use a compass or a nail and string to draw a circle, centered on the X, whose circumference is equal to the edge of the board - the arc of the circle should touch all four sides. Using a triangle or something similar, draw lines perpindicular to the "X" lines tangent to the circle - they will form the four "corners" of the octagon. Slice off those corners and you should have an equilateral octagon.