The specifics will be determined a little by the RIP you're using. Still, you need to create a Spot Color in a color palette called either CutPath or CutContour...and it usually has to be spelled exactly as shown. You do this under Tools, Options to get to the color palette editor I believe, to add the new color. Any color will do, I use a Magenta derivative so it's easy to see. When using it around an image, it needs to be an Outline with a hairline width.
You can define it in several ways. X3 has a new feature intended to do this very thing called Boundary, though I wish they would have given you some options for helping to define some bleed into the image or offsets. Other ways are with the Interactive Contour, then applying an Outline with the hairline thickness to the Contour.
