Stele captures content directly from your browser, so the extension is the starting point.
The Chrome Web Store listing is still in review, so the extension is installed by hand for now: download the build, then load the unzipped folder in your browser with developer mode on. The install page walks through it step by step and explains the developer mode prompt Chrome shows afterwards.
The same steps, and the download link, are in the app under Settings → Extension.
Once installed, the web app detects it automatically and unlocks capture features. The web app works without it too. You just won’t be able to capture from a browsed page.