I drew the symbols in Autocad and printed them out on white waterslide decal paper on my plotter.
Cut them out and placed them on the buttons using some decal set so it helps you remove wrinkles and helps glue them down better.
Here's what they look like after gluing them down and folding over the edges. The black on the edges might get rubbed off in some places but a sharpie takes care of that problem.