That's a great idea about the ribbon. I think you could sew it along the bottom of the pocket and then have it curl under and tuck in.
Pocket: cut one piece 2" by the approximate height of a pen/pencil (but shorter than your journal)
Backing: cut two pieces 2" by the height of the notebook + about 4" so that the elastic is only on the inside cover (one of the pieces that is scene is the "front" backing piece while the other piece isn't scene at all so it's the "back" backing piece)
1. Fold the pocket top under twice to cover up the raw edge. Sew it to the front backing piece -- the wrong side of the pocket against the right side of the front backing piece. (test it on the journal to be sure that the pen will line up where you want it to).
2. Sew the the front backing piece to the back packing piece, right sides together. Just sew along the long sides.
3. Turn inside out.
4. Press the edges under 1/2". Stick a piece of elastic in and sew the casing shut.
LOTS OF PRESSING
If you do a patchwork front like mine, then you should iron a piece of interfacing or other fabric to it so that the pen doesn't get caught on the patchwork.