I used 1 inch wide elastic although you could use wider for a larger pocket. It needs to have some overlap like I've demonstrated in the picture. The final circle needs to be small enough so that it fits on the front cover snugly.
For this project, I cut the elastic 21 inches long, so that it would fit a large Moleskine (8 inch x 5 inch).
Step 2: Pinning the elastic
Next, pin the elastic so that there is some overlap. The pins will help stabilize the elastic once you start sewing.
For this one, I allowed the two to overlap 5 inches.
Step 3: Sew the elastic
Now, sew the elastic on three sides. I've added dashed lines to show where my stitching is (the black thread didn't show up very well on the black elastic). This will create an opening at the top for you to insert the pen.
Step 4: Affix it to the Moleskine
Viola, now you're done! Slip it over the front cover of the Moleskine. I like using a Space Pen, but any pen will do. You can also check out the side view.