We suggest that you upload a PDF file where the pages are organized as follows:
One side (page) per page in the file. In other words, do not upload spreads (that is when you have, as an example, pages 1 and 4 of the book, as part of the same digital page)
If you chose DOUBLE SIDE PRINTING, even if some sheets get left blank intentionally, your file should include those blank sheets.
When you upload your file, you should be able to get a preview. Let us know if you need further help.