Create a new file ./backend/.env by taking ./backend/.env.example as example.
Install depencies by running yarn install at the root of the project.
Test by running yarn start.
It should open 2 new tabs in your browser, which must be sync.
You can test it without running locally. It's hosted on free tier heroku so you it should take some time to warmup.
Website 1 plugged on Backend 1
Website 2 plugged on Backend 2