I learned wiring a guitar pretty much the same way I learned setting up a stereo: first of all you need to figure out where the signal comes from (i.e. pickups respectively CD, tape, tuner, phono) and where does it need to go (output jack respectively loudspeakers). Everything else can be deduced from there.
Taking a picture is a good idea, so is drawing one if you don't have a camera.
Cheers Stephan