These are the two steps for formatting a book recommended by book promotion services experts.
1. After periods, use a single space.
Retrain yourself if you were taught to type two spaces after a period.
Use a single space in the digital age. (See "Space Invaders" for further information.)
Use your word processor's Find and Replace tool if your document has two spaces following a period. Type two spaces in "Find," one space in "Replace," and then hit "Replace All" with reckless abandon, typing teacher be damned.
2. Double-spaced line spacing is recommended.
This can be the most profitable thing you can do for your editor.
Give your words room to breathe on the page if you want them to see every last Word.
If you've already written your book with different line spacing, select all of your text in Word, go to Format > Paragraph, and then choose "Double" from the "Line spacing" drop-down box.