1) The easiest way to do this is to take an envelope or a bag of some sort, and glue that on the page.
You can use these as extra pages or "tip-ins" (Tipped-in page is a page that is printed separately from the rest of the book and then added to the book as a page; either bound in together with the other pages, glued in or glued on an existing page.)
It's fun to use envelopes with windows or plastic bags or translucent bags.
There are also printables online, like library pockets. Those are kind of cute. :-)
2) You can also glue a pocket in by adding glue or double-sided tape on the edges of a page, or tape it on place.
You could also sew the pocket in place, or staple it. Any way you can attach the pocket on the page is good. (Sewing and stapling adds texture, if you use washi-tape, you add colors and patterns.)
It doesn't need to be a rectangle, you can use any shape as a pocket.
3) Glue two pages together at the edges, leave the top open. That is sort of a hidden pocket.
You can also leave the side open.
Or you could glue the pages together at all sides but leave them unglued in the middle, and cut a slit in one of them to create a pocket. It might be a good idea to glue the top of the slit on the "background" page, to make it easier to use.