1. Print monochrome converted photo on paper.
2. Take printed photo outside to nearest dirt pile.
3. Rub photo, face down, into dirt pile. Crumpling the paper a little helps.
4. Take a paperclip, needle, nail, or other sharp object. Carefully scratch the pointy end across the photo.
5. Finally, spray a fine mist of water onto the photo. It doesn't have to be evenly coated. Leave it in the direct sun to dry.
6. Scan the photo and post to Facebook as a "Throwback Thursday" shot. A nice heart-warming story to go along is a good touch - no one will be the wiser.
I've seen a lot of people just convert to monochrome/sepia, add some vignette and call it a day. There may be some textures that you can merge onto the image, as well.