Believe it or not, I use Excel. I've made templates of switches, pots, etc. and copy them into a new worksheet, then use different colored lines for the wiring.
It's just a drawing tool, and I'm not doing real schematics with it.
I'd just use whatever paint program comes with your OS. Most programs that do that kind of thing any better probalby cost more than you want to spend. As long as you can draw rectangles, circles and lines you should be able to make fairly neat schematics.
i had to get a program called electronic workbench a couple semesters ago for my associates degree electronics program
the program has many many things for making circuits you can run them hook o scopes in multimeters you name it you can probably do it. If all you want is guitar diagrams however this is extreme overkill
I use MS Paint, I create a template or cut 'n ' paste some on occasion, then I select from those ... most of the guitar wiring is not put into symbolic form, either that or I'll create my own pictorials for say rotary switches ... I have a couple EE programs that are used for various things, bode plots, frequency graphing, pup / tone circuit modelling, along with more complex stuff.
For just guitar wiring, just grab whatever graphic program you want ... you can always *box* components, and just label their functions, then show connections as needed.
well, it took a while but i just drew all of the switches and pieces of electronics i could think of in ms paint, i'll just cut and paste em in ms paint to make it work