Before you do that, though. Let's make sure that you are ready to proceed.
1. Your book's page-count needs to be a multiple of 4 pages (e.g., 4, 8, 12, 16, etc), and up to 72 pages total.
2. And your PDF File needs to be set up as a single-page PDF file with 2.3mm offset crop marks and 4mm bleed.