Navigate to the root of your theme in Terminal.
1. Installing grunt globally
npm install -g grunt-cli
I ended up preceding this with sudo
to get it working.
2. Move Gemfile, Gruntfile.js and package.json to your theme's root
3. Install gulp (globally) and gulp-cssimport
npm install gulp
npm install gulp-cssimport
4. Install required packages
npm install
5. Run bundle install
bundle install
6. Run grunt
grunt