Step 1: Gather Your Tools
Thin ribbon. Make sure it's long enough to run the length of the diary and poke out some at the bottom plus 3/4 to 1 inch allowance for securing the ribbon to the spine of the diary. I've chosen the silver satin one on the right -- it used to be one of those dress pants hanger loops. Hooray, I'm re-purposing!
All-purpose adhesive that you can use on paper and cloth. I used UHU glue.
Thin flat instrument that fits in the space between the outer cover and the spine of the diary. Mine is a rather thin diary, less than 1-cm thick, so I used one of the blades of a thin pair of scissors.
Step 2: Open Up Some Space
Slot the scissor blade (or your instrument of choice) into the narrow space between the outer cover and the spine of the diary, to open up the space.
PS: I'm showing this on the bottom on the diary, because this Instructable was made after I'd done everything. ;-P
Step 3: Glue and Slot in Ribbon
Spread a thin line of UHU glue to one side of the 3/4 - 1 inch allowance on the ribbon.
Glue side facing the paper spine of the diary (not the cover), carefully slot the allowance end of the ribbon into the narrow space. You can do this with just your fingers, scissor blade or instrument of choice to help push the ribbon in. Be careful not to scrape off any glue onto the top of the diary pages. If it happens, just quickly wipe it off, before the glue dries.
The glue should still mostly be wet, so you can use the scissor blade to push and slide the ribbon down the spine to the desired depth.
Use the scissor blade to press the glued ribbon surface into the spine to ensure it sticks.
Step 4: Trim Bottom End and It's Ready to Use!
Hooray, now my diary is complete with a placemarker!