Before usage you must have node.js with npm (node package manager) installed.
First install grunt as a console service. Run
npm install -g grunt-cli
This will install grunt in your system environment. You can read more here http://gruntjs.com/getting-started
After grunt instalation change to your project dir where package.json is located. And run the following command.
npm install
This will install grunt, which is used for LESS compilation into CSS. Run your grunt while development for auto-compilation on any .less file change in /less directory
grunt watch
You can also run grunt watch in windows by grunt-watch.bat file. That's it.