The kit provides a simple way to make interactive prototypes that look like pages on GOV.UK. These prototypes can be used to show ideas to people you work with, and to do user research.
You may already have it, try:
node --version
Your version needs to be v4.4.7.
If you don't have Node, download it here: http://nodejs.org/.
Unzip the file
Open a command line app (Terminal on OSX) and change to the unzipped directory. Then run:
npm install
This will install extra code that the prototype kit needs.
node start.js
Go to localhost:8080 in your browser.
Any code changes should update in the browser without you restarting the app.
The app recompiles app/assets/stylesheets/application.scss everytime changes are observed.