Use a sheet of wrapping paper or similar as large as you want your octagon to be. Make sure it is a square and trim if it isn't. Now that you have a square piece of paper, fold it in half to form a rectangle, fold the rectangle in half again to firm a square, fold the square in half to form a triangle. Once you have the triangle, measure out from the point and make a mark on the straight edge half the distance of your finished octagon, mark the second straight edge the same way. Use your ruler to connect the marks and cut off the extra paper on the mark. Fold out the paper for an octagon.