What I use to do years ago, before most apps had auto crop marks. Was create the guidelines for the margin. Use a line tool, pen tool, etc. and create a line that goes from the guidelines where they meet each other and draw outwards to the edge of the page. Because the line is on the guideline, it snapped to it, making it a straight line. Repeated for the same corner drawing the other direction. (In other words, if I was doing the top left corner, I would draw out to the left, then repeat to draw out to the top of the page).
Group the two line, copy and flip to snap it to the next corner and repeat until all four corners of the page are done.
But if you have a postscript printer, acrobat pro, adober illustrator or adobe indesign, they will generate the crop marks for you.
tip: Do the above while creating an action and your done for the next time. Just run the action. But I reccomend that you set the rules to percentages when creating the guidelines, that way, it can work on more than one image size.