Under the File menu, choose New Document In the Width and Height put the dimensions from the publications Trim specifications. Fill in the bleed information from the bleed spcifications.
Check The Colors Used
Select the Swatches Pallete in Illustrator 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 Save As...
Select Adobe PDF in the Format option, as shown below.
Name your PDF file and click Save.
Once you have named your PDF file, you will then be prompted to select the PDF settings.
Select [PDF/X-1a:2001] from the Adobe PDF Preset option, as shown below.
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 Save PDF button.