Where do I make the full KDP paperback cover (back, spine and front) for my book?

From version 1.9.22, open the book's page and, in the Covers card, press "KDP print wrap". It opens the Cover Studio with the full paperback cover already set up for this book: its front and back covers, its trim size and its page count, which sets the spine width. The page count is the real one when you have exported the book as a PDF at its current trim and text size; otherwise it is an estimate, so export the PDF first for an exact spine. The button needs a front cover first.