There are various ways to find the center of a circle.
Here is a simple method that seems interesting. Place a
corner of a piece of paper on the circle as shown and
mark the points where the edges of the
paper cross the circle. Turn the paper about 1/4
turn and likewise mark two more points. Lay
a straight edge across the first pair of
points and draw along the straight edge. Do
the same with the second pair of points.
You have drawn two diameters which cross at
the center.