Ensure that your dashboard is in "Edit Dashboard" mode, then click the ellipsis "..." icon on the widget that you just added and select Edit.

On the Edit Widget window on the right hand side, go to the Data tab and select the Google Sheet from your Google Drive account that you'd like to use with the widget.

Then, select the spreadsheet tab and cell from that sheet for which you'd like to pull in data.

Finally, configure any additional options, such as widget title, color, and theme. Click the "Save" button to save your dashboard.