Just trim the sheet to two pieces of 6x12 and fold up a 2" edge on each piece, then fold them evenly into 3-panel accordions. Back the two pieces together with the folded edges on the outside and tape the top, bottom and sides.
Fold and stick a long strip of duct tape to itself to make a 10-12" band to hold the wallet shut with velcro. Embellish the top of the band with some buttons. An ATC wallet is a super cute way to carry your cards to and from a swap. Use two sheets of cardstock and tape the pieces together at the ends to create a longer accordion fold to hold more cards.