Print the 3 pages out at 100%, which will print the cards at the same size as a standard Magic card.
Each character should have a zombie as its backside, and the Shambling Hordes and Anti-Zombie Serum should go front and back together as well.
So, you will end up with 13 cards:
(12) Character cards, with a Zombie on the reverse side
(1) Shambling Hordes/Anti-Zombie Serum card