https://guide.elm-lang.org/install.html
You can download the Elm installer here or just install it using npm:
npm install -g elm
Then create a file called Pong.elm.
In the same folder, run this command in your terminal:
elm-reactor
This will start start a local development server at http://localhost:8000. When you make a change to your .elm file your can just refresh your browser and Elm will re-compile your file.