Get a box of colored pencils.
With a regular lead pencil, draw a box on a sheet of paper for each and every component on the board. Label the boxes with the id of each component it represents.
Pick one box and with a colored pencil draw all the connections from that component to every other component. Lay that color aside.
Pick another color and begin to draw the connections for another box.
When all done, draw a proper schematic using standard symbols. All connections needed for the schematic should be apparent from your colored pencil drawing. Where multiple colored lines connect components, you know only a single board trace connects them all.
Paul