Load your core by clicking on the first tab. Scroll down until you reach the desired Core. We will use Nestopia for now. Don't forget - Content must be compatible with the matched Core.
etc.
After selecting Core, click Run. After RetroArch opens, click Add Content and select your compatible ROM.
etc.
Download/upload/erase save data
If the Web Player doesn't start, you should click "Delete all" and refresh the cache in your browser (usually F5 or Shift+F5).
If this happens, please also report an issue on GitHub with logs from the browser's developer tools console.
If you click on the three line icons, the Quick Menu will open in RetroArch.