In Illustrator, select File>Open, choose your PDF cover file. It will open as a new document at the correct size.
Convert the document space from RGB to CMYK (File>Document Color Setup>CMYK)
Lock the template layer to avoid any accidental shifts, then create new layers as needed to work on your art.
Save OFTEN (as an Illustrator document).
When you’re ready to save a PDF, hide the template guides, then SAVE AS a PDF/X4:2008.
We always recommend doing an A/B check between the blank cover PDF and your finished file PRIOR to uploading- they should have the exact same size in Acrobat. If they do, you SHOULD be in great shape!