These are the recommended steps to ensure you file will pass preflight the first time if you do not follow these steps you may have posision your ad in the Ad portal
Under the File menu, choose Document Setup and ensure that the dimensions are correct.
If needed, change the document size and move the ad file to the middle of the new document.
Ensure that the trim and bleed sizes are correct while you have the file open.
Ensure that all marks are correct and that they are outside of the image area.
Check The Colors Used
Select the Swatches Pallete in InDesign and ensure that all colors not intended to separate as a spot color are set to separate as a process color equivalent.
All colors not intended to separate as a spot are defined as CMYK and the Color Type is Process.
Scroll through the colors and see if the intended spot colors are set to spot and the intended ones are converted to process. Double-click on a color to edit it.
Save The PDF File
Under the File menu, choose Adobe PDF Presets and select [PDF/x-1a:2001].
Name your PDF file when prompted in the Export pallete and click Save.
Once you have named your PDF file, you will then be need to export the PDF with the PDF/X-1a settings.
Select "Marks and Bleeds" On the left. Set the Marks to "All Printer's Marks".
Set the Bleed to the Specificaton amount.
Simply click on the Export button.