After we finished the front, they bound it to the inside sheets by sewing it together with a sewing machine (sorry I don't have a picture). It was pretty thick, with 10 or so sheets inside. The trick was to slice the raw edge after to create the finished look. (or all the pages inside would stick out farther than the brown card stock in varying lengths)
Definitely a fun project, I am glad I had help making all 75 of them!