How to proceed to draw the circuit diagram of an unknown circuit?
I usually take a picture of the top and bottom of the board. I line the photos up in software and switch between the top and bottom pictures. Then draw the traces over it in a program that allows stacking of images and vector graphics (power point, visio). 

If the program has layers, then traces can be created for both top and bottom layers. I then identify each component, either by visual inspection or by unsoldering and using a digital multi meter to find the functionality. 
 It's also nice because it allows one to label all of the components. This would be difficult to do on a four layer board at which point, instead of taking pictures, an x-ray machine would need to be used. 

The picture below shows this done for the top layer of a board, which all of the traces on the top layer have been traced out. If I couldn't see a trace, I used a microscope to find out where it went and sometimes a continuity check.