Cut two pieces of elastic that measure the full length of the cover. The elastic will be glued to the outside of the cover (on the side with the pockets).
Place a piece of elastic 1 inch from the top edge and place the other 1 inch from the bottom. With each piece of elastic, start by gluing one end to the midpoint of that side of the cover. Once the glue has set, bring the other end around to create a loop, and glue that end in place.
When you are finished, you should have two big loops. These elastic loops will wrap around the wallet and prevent your belongings from falling out. They can also be used to secure a boarding pass to the outside of the wallet.