Put your pages the faces of the cards on the table. You should not be able to see the cards.
Fold your pages in half. The ends must be perfectly aligned. The back and front of the cards should meet.
Unfold your pages. The layout is ready to easily fold them again.
Apply some glue on the blank part of the page, under the back of the cards.
Re-fold the pages as you did before.
Cut off your cards.