Draw a circle first, and draw a diameter AB through the center of the circle. Then, use compasses to make two points of the circle where the middle perpendicular of the diameter AB intersects, that is, C and D, that is, AB is perpendicular to CD, and use the middle perpendicular of AC, and the two points of the circle intersect are E and F respectively.
The middle vertical line is AD, and the two points of the intersecting circle are G and H respectively.
Then the regular octagon AECGBFDH divides the circle in two.