Here is how...
Fold the bottom of the envelop to the top of the envelop.
Fold carefully so that the flap of envelop is still able to be closed.
Use scissors to “shave" along the three folds that I have highlighted in the picture. (This is by far the most challenging step. I am a huge advocate of "hands-off" moments in my classroom. Take your "hands-off" your supplies, and have your eyes on me.) By shaving those three folds, you now have an additional page for gluing your flippables and foldables. It is important to not shave past the fold that was made in the first step.
Carefully use the smallest bit of liquid glue to seal the fold from Step #1. This will allow you to still have an envelop for students to store notes that you do not glue in place.
With a well-placed single hole punch, you can close all the pages of the
Fantastic Math Cheat Sheet Organizer with the envelop flap.
BAM! Decorate, add all the flippables and foldables that create in class, and enjoy!