Step 3: Make Cover and Glue Accordions on One Side
Cut a piece of cover material as wide as the widest ("tallest"? - check the picture) card you will store. For inside gluing, make it, say, another 1/8" wider. Cut the piece about 3 times the length of a business card (front, back, top flap).
Previously, when fanfolding either the 0.25" or the 0.20" quadrille graph paper (of length 11"), you probably ended up with an accordion with one extra flap, kind of like this: /\/\/\/\/\/\/\/, like an "N" with too many middle bits. Cut one of them off so you end up with something like this: /\/\/\/\/\/\/\/\, like an "M" with too many middle bits. This is more or less optional for the inside method, mandatory for the outside method.
Inside: Glue the outside of the last flap of an accordion and align it with the side of the cover at one end. Repeat for other accordion on the other side of the strip, width-wise. If you need to, Place a card to be stored into the compartment closet to the cover material to check for fit. Make sure the accordions are "mirrored", that is, that they create symmetric "compartments" between them.
Outside: Glue the inside of the first flap of an accordion and place it over the cover material at one end. The material itself and the fold help to get perfect alignment. Repeat for the other accordion on the other side of the cover, width-wise.