Pre-requisite: Elm 0.19 (https://guide.elm-lang.org/install.html)
To build the app, you need to compile src/Main.elm to public/arklay.js. Do this with:
elm make src/Main.elm --output=public/arklay.js
For development, you'll probably want to add the --debug flag, but if you're finished development and want to upload, use --optimize.
For a live-reload environment, you can use elm-live. Install it globally with npm install -g elm-live, and then start it with elm-live --dir public -- src/Main.elm --output=public/arklay.js --debug