Node >= v8.9.0
npm install
cd into your project folder and type the npm run dev command into terminal, to start the development server and Gulp. Eleventy has baked in hot reloading and will files for changes.
For those of you on Windows, see Daniel Schildt's potential fix for the environment helper.
Type the npm run build command to minify scripts, styles and run Purgecss. Files will be available in /dist