Searches on the net resulted in a couple of pages that provided tutorials on how to go about making these pockets. You can see them here and here.
Thought I'd show my version and how I go about making them..
To me, making pockets seems mostly like just applying different folding techniques, and you might find ironning at every other stages helpful.
The finish completed bellows pocket in my Odekake bag.
First, cut your fabric depending on the number of pockets you were making. Here, I'm making two pockets, so I doubled my usual 18cm x 12cm plus 1cm seam allowance all around. You could add light iron-on fusible interface (learn more about fusible interface here) to give the pockets a sturdier shape. However, I didn't fuse here *gulp*
Next, turn and fold, I double fold the top, and fold once for the sides and bottom. You could serge all around to avoid fray, as you can see I don't serge *roll eyes*
Sew the top part - I double stitch the top.
Next, fold the fabric in half and press, open and fold about 0.5cm on each sides of the centre line. Adjust the size depending on how deep you want your pockets to expand. (Note: I don't really take a ruler and measure, I just use common sense *yikes*)
Do the same for the sides as well, now you have 4 folded edges creating 2 pockets. Sew along those edges, you will now have 4 crisp edges.
If you need to add labels or any other embellishments, do them now. Next, position the pocket and pin all around.
Sew the centre line creating partition to the 2 pockets.
Sew the sides next and then the bottom part. And now you have a nice expandable pockets for your bag.
This is how my pocket sides look like.
Please go easy on me as I'm a newbie.. I would really love to hear your point of view, tips and suggestions on making bellows pockets in particular.