The first step to get this running is to install the dependencies specified in package.json. You may do so by executing npm install in a shell. You may also be able to use Yarn.
At this point, you should ideally have set up the Mercury Backend. If that is running on a remote server, you should set up ngrok like I've described here and set the link provided in the MERCURY_BACKEND_HOST constant in src/App.js. If not, just set the MERCURY_BACKEND_HOST constant in src/App.js to the url that Mercury Backend is running on. This can also be set with the REACT_APP_MERCURY_BACKEND_HOST environment variable.