Sean McCambridge, depending on your version, it might vary. Basically using the Pen tool, you draw or trace the route you want. Be sure your image is at zero,zero when you create a path, otherwise it will be offset when you put it into Raphael.
You can Export or Save As… SVG. Somewhere in the code there will be an element called . The text inside the d attribute describes you path.
Your mileage may vary, but that is the quickest and easiest way to get the encoded path. You could read-up on how it all works and do it by hand. Once you get the hang of it, for simple paths it isn’t hard. Understanding SVG paths could be an article in itself!