After getting your image to its print size, you can just go to image/canvas size if you check the relative box and set up the height and width to give you the borders you want, have the anchor in the center, if height and width are equal, you will add an even border all round.
I use this method with an added step of adding an extra size of the border at the botom by again using the image/canvas size and with relative checked set the height to half used in the all round border and set the anchor to top centre.
Peter