From all the research I have done you are allowed to do it any way you want, Artistic licence. Pen, pencil, on the print, on the mat on the back or any combination, all are acceptable. I use photo shop to title the image in the lower center using middle gray (RGB 127, 127, 127) Then with a grey or silver gel pen I number on the lower left and sign on the lower right of the print. This way the print is numbered, titled and signed. The gel pen ink is similar to inkjet ink and is very compatible with the photo papers. I found pencil hard to write on the print.
Marc
