First, are you sure you need to? I assume this is related to the old word count thing about counting words at 250 words per page.
If you still want to do it (at least in Word 2003):
Highlight the text (if you already have text)
Go to: Format > Paragraph
Go to Line Spacing. select "Exactly" on the dropdown menu
Go to the "At" dropdown and remove what's there (place your cursor in the box and hit backspace). Key in "25" in the box (NOT 25 pt, just 25)
Click OK
Be sure your margins are set correctly.