Open your document in Microsoft Word
Select the text you want to double-space (or press Ctrl + A to select all)
Go to the Home tab
In the Paragraph group, find Line and Paragraph Spacing
Click Line and Paragraph Spacing
Select 2.0 (Double)
If needed, open the Line and Paragraph Spacing menu and choose Line Spacing Options
Set Line spacing to Double
Click OK
To make it the default for new documents: Home tab → Line and Paragraph Spacing → Line Spacing Options → Set as Default → OK