1.) Begin with an arbitrary square. (To see the construction of a square, click here.)
2.) Find the center of the square by constructing the diagonals and marking where they intersect.
3.) Construct four circles. Each corner of the square should be the center of one of the circles. The radius length should be equal half the length of one of the diagonals.
4.) Construct a point every where the circles intersect the original square. Connecting these points will make up your octagon.
5.) If we hide the path objects in GSP, we see the following:
Click here to see this construction in GSP