Open your Google Docs document.

Highlight the text that you want to set to double spacing.

Click on the "Format" menu at the top of the screen.

Hover over "Line spacing" and select "Double" from the dropdown.

The selected text will now be double-spaced.

To set double spacing for the entire document, press Ctrl + A (Windows) or Cmd + A (Mac) to select all text, then follow the same steps.