You'll need two binder clips, each the small kind, one Moleskine or other notebook, and one pen of your choice.
Step 2: Clip
Take your binder clips, open the Moleskine to the back cover, and carefully apply the clips to the cover, right over the pocket. They won't go down far enough to interfere with the pocket. You can place them lower or higher depending on the size of your pen.
Step 3: Clamp
Secure the clips by flipping them over. You can either leave the metal bits there, if you think you'll be removing the clips often, or continue on to the next step.
Step 4: Remove Handles
For a more streamlined look, remove the metal handles by pinching them towards the center. This will draw them out of their holes in the black part. Do this for all four metal clips, and your project is complete. All you have to do now is adjust them depending on how thick your clip is.
Step 5: Add a Pen
Whatever you want. Works best with flat clips. Adjust them slightly so that the pen fits as snugly as possible- you may leave some slack, if you plan to pull the pen out entirely. Or, you can leave the pen cap where it is, and avoid losing it.