This is not a detailed document on how to use Inkscape in general- there are other resources online for that. It's just enough to format things for K40 Whisperer.
You can possibly use other design software to do your design, as long as it can export to an SVG. Because K40 Whisperer is pretty tightly integrated with Inkscape's SVG files, you may need to open and re-save in Inkscape to get certain properties right.
Inkscape/K40 Whisperer on macOS:
This is somewhat tricky to install correctly. It requires a built-from-source version of Inkscape (I used Homebrew, though the version on there has a broken dependency. I then had to hand-apply a patch that hadn't been accepted yet.), and a number of Python libraries. You also have to run it via sudo, as PyUSB can't open the serial port as non-root.