Usually only one valve appears at first, sometimes more. You need to turn on FOUR valves by walking up to them and holding x. They are scattered around the sewers all over all the way up to the entrances to certain areas of the map and even all the way down near the decon pad. You will find cream/brown pipes all around the sewers (if you're unsure which they are there are four of them on the walls of the tunnel between the sewers and value voltage). Opening the debris on the way to the Atlas Command first is a good idea as the 3dprinter will always spawn in there first. Remember to keep 2 or 3 zombies alive at the end of the rounds to allow you to more efficiently complete the steps without having to worry about killing. There are no variations between the solo and co-op method. However I have still included the steps in detail below. The youtube video at the bottom of this guide is all you should really need, To avoid this glitch do NOT rescue any survivors throughout this easter egg. Until a patch is released this achievement MAY glitch for you.
