Kill the zombies. Shoot the cannon with space (only one projectile at a time (still)), press Z for the (not so) autocannon, press R for the rocket (this time the rocket lasts forever). Press Z to disable the rocket on the first wave (I don't know why you'd want to do that though).
I just took the script of Sole Survivor and changed it up a bit. Nothing special there. I will be remixing some things and changing them a bit.
