The Sheet ID is the long code in your sheet's URL: docs.google.com/spreadsheets/d/THIS_PART/edit. Sharing must be set to "Anyone with the link — Viewer" (Share button, top right of your sheet) or this will fail to load.

In Google Sheets: File → Download → Microsoft Excel (.xlsx) or Comma Separated Values (.csv), then upload that file here. No sharing settings needed — nothing leaves your browser. You'll need to re-upload after you update the sheet, since this won't auto-refresh.