Need advice on "best" way to prepare book cover for printing

Question

This newbie has created a front cover, back cover, and spline in Photoshop Elements with multiple layers consisting of a background image of a mountain, text, and on the front cover, an airplane. I've combined the three parts into a wraparound cover within a single indd file, which I've exported to PDF for upload to Amazon's CreateSpace (CS) for a print-on-demand book.
CS instructions say to "flatten" the cover. I've researched the term and found forum discussions that mention "raster" and "vector" and the pros and cons of different options for preparing a layered image for printing. I'm aware of the cautions about flattening being an irreversible process, and to never do that to a master copy. Beyond that, to say I'm confused would be an understatement.
I realize this is an InDesign forum, but I'm seeking advice on whether there are actions I should take in Elements before placing the three parts of the cover into the InDesign document for converting to PDF. For example, should I flatten them within Elements before placing them in the InDesign document, then convert to PDF? Or is it better to do that on the complete cover within InDesign?
Any assistance on clearing up this fog would be appreciated.

Answer 1

The print-on-demand site's instruction to flatten really has no direct relationship to InDesign, Peter. They don't say anything about what application you should use prior to creating the pdf, and that's what generated my original plea for help as to whether it would be best to flatten the individual images within PSE before placing them into InDesign, wait until they are combined and then do it, or if it is even necessary to do that at all.
I've been doing some additional reading on the CreateSpace forum and found a couple of very detailed (translation: too detailed for me) discussions on the differences in the output based on how you tell Adobe to create the pdf. I also found the following buried in the Help section:
                  Check the PDF settings that will be used to create your PDF file. If available, select "PDF/X-1a," "High-Quality Print" or "Press Quality" from the list of presets. If your system allows, make sure the following settings are chosen. Any other settings should be left to default in most cases.
                  Fonts and images are embedded.
                  Bookmarks, annotations, and comments are disabled.
                  Document security (any type) is not used.
                  PDF/X format is used. PDF/X is preferred, but if you are submitting non-PDF/X files (for example, PDF/A), any comments, forms, or other non-printing objects could be removed during our review process.
                  Transparent objects are flattened.
                  Spreads and printer's marks are disabled.
                  Downsampling, or decreasing resolution, of images is disabled.
                  Bleeds are enabled (if applicable).
                  Click "Export" or "OK" in the export dialog box.
                  Once created, make sure to open the PDF file to see that it appears as you intended. Otherwise, make the necessary adjustments in the native document and re-create the PDF file.
The mention of "High Quality Print" in the first line and the fact that it was InDesign's default setting for export left me with the impression that I didn't need to do anything other than export the cover and it would be in compliance with the requirements. Then when I read on the other forum about what each of the settings does to output, and all this discussion about which setting is best, I wasn't sure if the comment about flatttening applied to my output pdf or not.
As for bleed, yes, and I hope my math is correct in calculating the spline width for cream b&w paper, 374 total pages, trim size of 5.25x8, and a bleed of .125" all the way around, which increases the InDesign document by .25" in width and height over the trim size. I fully expect to have to do all that again, but such is life in the fast lane.