Open the Google Doc
Select the text you want to double space, or press Ctrl+A on Windows/ChromeOS or Cmd+A on Mac to select all
Click Format in the top menu
Hover over Line & paragraph spacing
Click Double
If needed, use the toolbar line spacing button and choose Double
To make double spacing the default, click Format
Hover over Line & paragraph spacing
Click Custom spacing
Set line spacing to 2
Click Apply