Most popular notebooks (Moleskine, Leuchtturm, etc.) include a pocket on the inside back cover. This pocket is convenient for receipts, notes or templates you might want to include. One item I keep on-hand is a 12-inch/30cm paper ruler from Printable-Ruler.net.
If you need additional pockets, here are instructions for two different types of pockets. One is a simple pocket that can be created from two notebook sheets. The other is a classical origami envelope.
This pocket envelope is convenient because you can place it anywhere within your notebook. You can also have multiple pockets, although the thickness of the notebook may get out-of-hand! All that is needed is two journal sheets (total of four one-sided pages) that are adjacent. In the example below, my pocket uses pages 53-56. The last page – 56 – is just the backside of the pocket, so it is available as a regular page.
For this Moleskine 3.5-inch by 5.5-inch art notebook, I measured 3 inches down from the top of the page (along the outer edge) and drew a diagonal line from the top of the notebook at the spine to the 3-inch mark. I then cut along this line using scissors. You could also cut this using a small cutting pad and penknife.
Originally designed to serve as an envelope for sending a letter, this folded envelope is handy for storing small items such as coupons, receipts and photos. Below are pictures of the envelopes I pasted into my notebook. For the 3.5-inch wide notebook, a starting size of 4.5-inches (square) works well. If you oriented the envelope as tall instead of wide you could have a larger envelope.
The front of the envelope (where you would normally write the address) is where you should apply glue and paste into your notebook.
I find these customized pockets to be advantageous for storing papers. A benefit of creating your own pockets and envelopes is that the materials stored in them are more accessible than in the standard pocket attached to the back cover. If you create these for yourself, send me pictures of your creations.