Write about what you'll will do after your bitten in the zombie apocalypse it can be when it starts or after it can be even about the first bite its all up to you.
Tag a friend in a friendly manor to encourage them to write what happens after their bit in a zombie apocalypse keep the story between 200 and 2000 words. Once your done try to tag three of your friends to continue the chain and if you can't well leave them blank. For the future chapters you can leave something that hints to a situation or just leave the name of the author you tagged that is up to you. Have fun!