This is simply a method to find the center of a circle, using very simple techniques. You'll need a ruler, a pencil and some way of measuring right angles.
You might want to use this technique to know where to drill the hole in the middle or draw concentric circles on the surface.
I can't take any credit for this as I probably learnt it at school, many eons ago. I just thought I'd add it because I saw lots of methods for drawing circles, using bits of string, wooden battens etc. but nothing for reverse engineering the problem.
If it's not correct then I can only blame my very poor memory.
Step 1: Draw a chord across the circle
Draw a line across the circle near the edge so it cuts the circumference in two places. This is called a chord.
If you can also make the chord a nice easy length i.e. 10, 20, 24, etc this might make life easier in the next step.