Slowly scrape the edges of the photograph using one end of the steel ruler. This will scrapped off some of the colours on the edge of the photograph to give it a rough white edges which makes the photographs looked distressed. It's easier to control the place to distress by using a steel ruler as compared to sand papers. Using this technique would also allow you to distress a small surface easily.
If you would liked another colour for the rough edges, simply ink the edges or chalk it. I've used this technique in my latest layout titled "Deep Within" in my family gallery.