You need node and yarn installed first.
Check .node-version for which version is required.
Install dependencies:
> yarn install
And start the app:
> yarn start
This will start the app and compile JS and CSS on the fly. You can open dev tools by hitting Ctrl+Shift+I or Cmd+Opt+I on a mac. Use Ctrl+R (Cmd+R on a mac) to reload the app.
Simply run:
> yarn package
This will build the app into the out folder.