Pass "Chapter1".
Pass "Chapter2"
Pass "Chapter3"
Pass "Chapter4"
Pass "Chapter5"
Pass "Chapter6".
Pass "Chapter7".
Pass "Chapter8".
Pass "Chapter9".
Finish the entire campaign.
Have a quick look at the credits.
Reach wave 5 while playing the zombie mode.
Reach wave 20 while playing the zombie mode.
Be ready to join the battle and start a new game.