I use a pen, on a sticker, on the back of the mount board. You can print whatever you want on the labels, or just hand write on them. I suppose you could do that directly on the back side of your prints as well. If I had to sign on the border, I guess I would use an extra fine tipped Sharpie. But honestly, I think signatures look horrible on the front (including on the board underneath the picture). YMMV, of course.