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From the context, you want to construct a REGULAR octagon.
This polygon has a central angle of 45 degrees, which is exactly HALF of the right angle.
The geometric construction procedure is as follows:
(1) If you have a diameter, you have a radius, too.
(you find the radius by dividing the diameter in two equal part:
- - - by bisecting the diameter. There is a special standard geometric construction for it, using a compass and a straightedge).
(2) Having the radius, you draw a circle of this radius.
(3) Then you draw an arbitrary diameter through the center, using the straightedge.
(4) Having the diameter drawn, you construct a perpendicular to it through the center, using the compass and the straightedge.
(5) Having this right angle, you divide it in two halves using the compass and the straightedge.
In other words, you bisect this right angle. There is a special standard procedure for it,
using the compass and the straightedge.
(6) Doing in the same way, you divide all right angles in two halves, drawing their bisectors.
Doing this way, you obtain 8 intersection points on the circle, that are vertices of your octagon.
The final step is to connect the consecutive vertices to get your figure.
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Below are the links to my lessons in this site, describing standard geometric constructions
- HOW TO bisect a segment using a compass and a ruler
- HOW TO bisect an angle using a compass and a ruler
- HOW TO construct a triangle using a compass and a ruler