The entire easter egg consist of doing challenges.
To start a challenge, you need to shoot the toilet roll next to the toilet. The toilet roll in front of the toilet will take the color of the challenge you chose.
I highly recommend you doing them from the upper one to the lower one.
Reward : Shit bombs
Reward : Zombie Shield
Reward : Max Ammo gobble gum
Once those 3 challenges done, you will have to achieve an ultimate challenge, consisting of 3 trials at once :
Regular zombies kill
Kill Zombies with poop bombs
Kill Zombies with the plunge
Once done you will be awarded with the ultimate plunge. If you plunge the galactic toilet, you will now get the ultimate toilet, the only change is that you can get to the final boss by ascending for 50 000 points.
The Ultra Shit is a real tank, to kill it you simply need to shoot a hell lot of ammo.
It can spawn shitty minion, so I highly recommend getting the Toxin-ZX1 to get rid of them.
Be sure to avoid the water since it can kill you really quick.
Once done, congrats you completed Poop of the Dead, now door will open, and you will have access to this entire doo-doo map.