How do you add narration if you already bought the book on kindle fire hd?

To add narration to a Kindle book you've already purchased on your Kindle Fire HD, open the book in the Kindle app. Look for the "Audible Narration" option, which may be available if the book supports it. If you have the corresponding Audible version, you can enable narration, and it will sync with the text. If you haven't purchased the audiobook, you may need to buy it separately to access the narration feature.