1. Determine the spine width first. Note that the spine width is relative to the thickness of the paper used for printing. For example, your book is of 300 pages, the spine thickness varies relative to paper thickness (say, 60gsm paper or 70gsm paper, or thicker art paper varieties). Get in touch with the Printer to give you a bound dummy (blank pages) with as many number of pages as your book has. Measure the spine manually from it. When you are more experienced, you would be able to calculate the thickness just by knowing the paper thickness (multiply by number of pages of the book).
2. Create a spread to represent the back cover (left) and front cover (right). Increase the width of either the left page or right page, to make it exactly the width of book + spine width. Use guides to separate the spine from the cover design area. Type the spine matter and rotate the frames to fit to the spine. Rotation of frames should be in such a way that when the book is kept on table with its front cover on top, the spine text matters should be erect (not be upside down). On completion the whole design should have the bleed area on all four sides, left page as back cover, spine in the middle with rotated text frames, right page as front cover.
3. You can also make the spine a separate page. Create a page exactly of the width of spine. Go to Pages panel fly-out and uncheck "allow document pages to shuffle". Arrange the three pages, the back cover the spine and the front cover in that order side by side in the pages panel (you can drag-drop the pages to make the 3 pages spread). Go to layout to make finishing touches.
Thank you very much!