With this guide I hope to help anyone that is wanting to do this Easter Egg. Either if its someone trying the Easter Egg for the first time, or if you're looking for specific parts of the egg that you are stuck on/(Chemical Step), I wish you luck and hope that this guide will help you.
The main Easter Egg will consist of 7 parts:
Part 1: Collecting Parts for the zombie
Part 2: Bringing the zombie to life/Turning the zombie into keys
Part 3: Collecting parts for the Chemical Step
Part 4: Getting the Color for the Chemical Step
Part 5: The Chemical Step
Part 6: Getting the bomb code
Part 7: Boss Fight
I split these steps into different sections so if you already know how to do certain steps but want help on another, you can easily access it.
This guide is very comprehensive and in depth for the Easter Egg. I tried to make it simple as possible, but also very detailed so you wont be confused on parts.
There will also be other sections on the side to help you with the quest.

(I HIGHLY RECOMMEND OPENING UP AND UNDERSTANDING AREAS OF THE MAP BEFORE DOING THE EASTER EGG IF IT ISN'T OBVIOUS ENOUGH)

I apologize in advance if it is hard to understand the guide. It is difficult for me to explain this through text and images only. This is also the first guide I have ever made.
Thank you!

This is Part 1 of the Easter Egg and where you will begin. This sections of the guide is mainly going over the locations and how to setup for the 2nd part.

(1st zombie part)
The first zombie part is an arm coming out from the ground on the beach (Holding the power switch). As you will notice when you pick up the power switch, it will lower back into the sand but will still be poking out. To retrieve this arm, you need to craft a buildable. Make your way towards the Ice Cream shop (To the right of the first door from spawn) and head to the back of the building. There you will see a blueprint. Grab that and place it on the crafting table inside of the back room. Then collect the 3 pieces to build the machine.

The first piece is on the ground, right at the corner of the building:

The second piece is laying on a bench to the side of the shop:

The last piece is near some portable toilets across from the shop:

Once you are finished with that, make your way to the beach and place what you just crafted on top of the arm that is peeking out of the ground. It will then start to rise and you will be able to take the arm.

(2nd zombie part)
The second part (2nd arm) is inside of a fire pit inside of the RV area, closer towards the motel.

(3rd zombie part)
The third part of the zombie is going to be our first leg piece. Make your way towards Elvira and right outside where bangbangs is going towards racing stripes, there is a tree on the right that is somewhat noticeable. Throw a grenade at the tree and it will drop the leg.

(4th zombie part)
The fourth zombie part which is the other leg can be retrieved by taking the cleaver from the main store and having it drop from killing the first green solider zombie with it. (If you kill solider zombies with other weapons, it wont drop. It will only drop on the first kill WITH the cleaver.)

(5th zombie part)
The Fifth zombie part is the torso. For this you will need the crowbar (which is inside of the motel right by bomb stoppers),

then you will need to collect a piece to repair the freezer trap inside of the main store. After repairing the trap, activate it then melee the pig that is covering the torso with the crowbar inside of the freezer. The part to repair the trap is across the street near the phone booths on the left of the gas station.

(6th zombie part)
For our final zombie part which is the head, you will need to unlock the portal to the projection room. To do this, you will need to collect Elvira's book and give it to her.
(To get the book you need to turn on the power, there is a section on the side that shows you how to do it)

This will then have her start talking and give you a vial to fill up. You will then need the cleaver from the main store and get zombie kills with it until it fills up. Once the vial is filled, give it to Elvira. She will talk again but this time will leave her chair and follow you. Lead her to the portal outline outside and she will open the projection room for you.
Once inside, interact with the buttons on the left side of the projection room and it will change the color to a greenish color. Leave the projection room after doing that and you will teleport inside one of the RVs. Right in front of you will be the final part for the zombie, the head!


Other parts:

Now that we have all of the parts to build the zombie, we need to collect 4 more parts to continue.
We need to first collect 3 mirrors. To get 2 of these mirrors, you will need the crowbar from the motel.
The first mirror (doesn't require the crowbar) is in Elvira's chair. You can interact and take the mirror when you give the vial to Elvira making her leave the chair.
The 2nd mirror is inside of the bathroom in the RV area where the sinks are. The mirror to the right you can melee with the crowbar and take the shard from the sink.
The 3rd and last mirror is on a car that is located in between the path by the gas station that leads to the beach, and the main store.
Now that we have the zombie parts and the mirrors, there is 1 last part needed to move on. The last part is a piece of paper on a desk inside of the back room of the Ice Cream Shop. The same room where you crafted the buildable and got Elvira's book.
This is the last piece you will need to collect to continue on to part 2.

After getting the zombie parts, the mirrors, and the paper, you will now use the pieces to receive the keys and move on to part 3.

Place the zombie parts on the green bed that is slanted upwards across from up-n-atoms.

Now place the mirrors,
The mirror from Elvira's chair will be placed on top of a green MRE box that is to the left of up-n-atoms.

The mirror from the bathroom in the RV park will be placed on the table to the left of the zombie

The final mirror from the car will be placed on this box to the left of the laser machine

Now that the parts are placed, walk to the small machine that is in between the laser ray and the boxes that you placed the car mirror on and put the paper in the machine.
Now interact with the machine and a yellow circle will start to move from left to right. You can start randomly clicking (Or if on controller, press A/X) to input a number. There are 5 numbers you need to get right. When hitting the wrong number, a red X will appear. With 3 red X's, you will be forced off but don't worry, you have unlimited attempts. Also if you get forced off but already got a number, when you go back onto the machine it will be the same number.
For example, you got the first number right but got kicked off the machine, when you interact with it again, that number is the same.
The circles represent numbers. So if you hit the 3rd circle, that is the number 3. If you get a red x from the 3rd circle, that means that digit is not the number 3, however can be the number of another digit. To make it easier, every number is different. So lets say 5 is the first number, that means 5 is not the number of the other digits. It may take a bit to finish this.
Once you get all 5 digits correct, it will grow green indicating you completed it correctly.
That will be a 5 digit number you will need to remember or write down. Once done interact with the machine and it will activate the laser ray being shot into the zombie, bringing it back to life.
Now you will need to go back to the little machine and interact with it again, doing the same thing as before, however you will need to enter that number you had guessed earlier backwards.
So if the numbers you had to shoot the ray was 27364 then the number you need to input this time as 46372. After completing this one, interact with the machine again and the laser ray will shoot for the second time. When it shoots the zombie this time however, it will turn the zombie into keys. Take the keys and you are done with this part, being able to continue on to part 3.
(You do not need the number from this step anymore, this step is just to get the keys to move on to the other part of the Easter Egg)

Now that you have the keys, you can make your way to the gas station and open up the garage.
Now that you opened up the garage, you need to find 3 parts to build your chemistry lab, and 3 parts to put inside of the bomb you will be making.
Lets start with the lab equipment.

Your first part is located in the back of the main store in between the store and the power area on a couch.

The second part is located on a bench on an incline from the beach leading up to the RV park

The last part is located inside the RV park in between 2 RV's where mule kick is, it will just be sitting there on a table.

Now you can go back to the garage and setup your lab
Now you need to get 3 parts to setup the bomb to be ready for the chemistry step
The first part is located under a wooden bridge on the beach, the first bridge coming from the water side

The 2nd part is located under a red car that is between the motel and the TV station
The last part is located on the ground tucked between 2 parts of the fence, by the TV station and across from the tree that you got the leg from earlier

You can now put those parts into the bomb
You are now ready for the Chemistry step!

Before we start the actual chemistry, we need to find our Solution, and the color for the boards.

To start this off, we need to figure out what solution we are going to be making. This part is simple. Kill zombies until you get a battery off of one of them via the bags. You now need to put the battery inside of a radio. The locations are here:

One behind the power station by some portable toilets, lying on the ground:

And one inside of a room that is in the motel. The room is by a set of stairs that goes to the TV station. The radio is on a couch.

Put the battery inside of it and listen to Dr. Bright recordings. You can keep interacting with the radio to play another voice recording. Keep interacting with the radio until you hear him say something positive about a solution that can kill the radioactive thing. An example that he will say to be the correct solution would be: "I devised the perfect solution to our mutated giant crab problem, (the solution)" Remember the solution that he says.
There are 4 possible solutions and here are what they can be:

1,3,5 Tera-Nitra-Phenol

3-Methyl-2,4-Di-Nitrobenzene

3,4-Di-Nitroxy-Methyl-Propane

Octahydro-2,5-nitro-3,4,7-Para-Zokine

(If you go through all of the recordings in one of the radios and he doesn't say anything about a way to kill the radioactive thing, then you need to get another battery and go to the other radio)

(NOTE: if you go through all the recordings and you heard him say that it was the right one but couldn't hear what he said, you can rewind the tape after going to the next round)

Once you know what solution is right to kill the radioactive thing, remember it and you can move on to now actually making it.

To start this off, we need to get our color for the film. This will give us our right numbers for when we do the Chemical part.
To do this we are going to make our way over to the TV station and change the color. It is across from Elvira.
When you change the color, you will notice that the whole game will turn into either red, green, or blue. Now that you have your color changed to whatever it is after 1 change, you can make your way to one of these locations.
1: Under the giant bridge on the beach

2: The gas station, to the right of the garage with the chemistry lab

3: Inside the back of the main store, to the left of where you got the one piece for the lab

3: The last location is inside of the RV, same one you got the head from earlier

You will see that all of these locations have numbers on them and an equal sign to it.
However you will notice that one of these will have a cross on the equal sign.
One of these is the number we will need to find our actual color. So in my game the one in the RV had a cross on the equal sign when the color is red, so that number is not it.
After you find out what number is crossed out in that color your in, you can stop going to that location and change the color again,
Repeat and check the locations until you find the one number that never had a cross on the equal sign. That will be your number.
For example mine was 8, these pictures is what your number should look like in every color.



Now that we have the number, it is important to REMEMBER THIS NUMBER/WRITE IT DOWN
Next we will make our way to the room that is in the motel aside the stairs leading to the TV station. Same room that one of the radios are in. You can look up above the couch that the radio is on and you will see another number.

This number you will need to multiply with the other number you got that didn't have a cross on the equal sign in any of the colors.
For example, the number I got in this room is 20, and the number I got that didn't have a cross on the equal sign is 8. 20 x 8 = 160. You will only need this number to find the color you will need. But always remember the number you got earlier mine being 8.
Next go to the TV station and walk to where Elvira is. You will see a TV by her with numbers that have greater than/less than signs beside them.
You will need to see if your number with the numbers on the TV is true. For example, on my game the number is 160. So on the TV, 160 is not less than 157 (Blue), and 160 is not in between 157-159 (Red).
However 160 IS greater than 159 (Green) which means Green is my color that I will need to change the game to.
Now that you have your color changed to the correct one, you can now make your way to these boards around the map. You will notice that they have ingredients on them and on the right of them are compound symbols with numbers. You will need to take a picture of these boards so you can access them easily. The locations are:
Outside the TV station by bang bangs

In spawn

Behind the main store facing the power station

On the beach near the water

In the RV park laid against the RV across from mule munchies

And the last one is located inside of the garage/chemistry lab (You don't have to take a picture of this one)

Now that you have all of these pictures you can now start the actual chemistry which is the next part.

Welcome to the Chemistry Step where you will add ingredients and make the solution.
So for this you need to know your solution that will kill the radioactive thing.
For example, we are going to say that the solution is 3,4-Di-Nitroxy-Methyl-Propane, as in my game.
We need to know how to make this step. This is where the cheat sheet comes in. To the right you can click on a cheat sheet section so you can do this step. You can check the cheat sheet while reading this section and follow along. The section tells you how to make the solution.
So for our example the solution is 3,4-Di-Nitroxy-Methyl-Propane.
According to our cheat sheet, the first thing we need to make is Formaldehyde. To make this we need to combine Racing Fuel and Quarters.
On the cheat sheet there are locations for the ingredients.
Once you get your ingredient, place it on the table that is in the garage. Place both of the ingredients (Does not matter where you place the ingredients, as long as they are on the table.)
Now that you have the ingredients, you need to find the number for them.
Use the pictures of the board that you took earlier and find the ingredients you have on the table on there. Here are the 2 pictures that have Racing Fuel and Quarters for the example.
Now that you see your ingredient on the board, you need to focus on the numbers to the right of the ingredient.
Lets take a look at the first picture of this example. This is racing fuel. There are 4 numbers on the right of it. You only need the TOP and the LEFT numbers. So for racing fuel, the top number is 5. The left number for racing fuel is 1. Now add both of those numbers. So the top (5) and the left (1). 5 + 1 = 6. That is the number for racing fuel.
Do the same with Quarters. The top (7) and the left (5). 7 + 5 = 12. Now that you have Racing fuel (6) and Quarters (12), you can add both of those numbers.
Racing fuel (6) + Quarters (12) = 18 18 is the number for both of these. Now that you have that number, you need to remember the other number from earlier that you got from changing the colors. The one that did not have a slash in the equal sign. For example mine was 8.



Now take the number that you got from the racing fuel and the quarters (18) and minus it by the number you got from the color changing. (8)
18 - 8 = 10 This is the number you need to input on the machine to the right.
After inputting the number go to the left of the machine where the dial is and interact with it. Doing it correctly will lead to your screen shaking then green smoke and other particles coming from the side of you, adding the chemical you made on the table to the right. If you get it wrong, it will red screen you and you will have to get the ingredients again and restart. As long as you do the math correctly you should be fine.
Doing that made Formaldehyde so next we would make Acetaldehyde which consists of:
Vodka and pennies. So using the cheat sheet to know that and find their locations, place them down and do the same thing.
Add the Top and Left number of that ingredient from the board. Do it for both ingredients. Then add both the numbers from each ingredient. Then minus the number you have by the one from the color changing and input it.
I will do this one as well for the example and then after, the cheat sheet should cover the rest.
According to the picture, Vodka is 7 + 4 = 11. The next picture says Pennies 1 + 4 = 5.
11 + 5 = 16.
16 - 8 = 8

Keep following the cheat sheet until you make the final solution. Once you make the solution, pick it up and place it in the bomb.
You can now continue to part 6.

This is the last part you have to do before you can start the boss fight, and is fast/easy to do.

You are going to get the crowbar from the motel area.

Then you will make your way to the back of the market, where the number is and on the side of the electrical area.
The desk inside of the room that I am pointing at, go up to the side of it and go prone to the side.
Look up and find the 4 digit number in white text on blue.
Remember this number
Now we are going to use this number in 4 different areas.
Our first area is here:
Behind the electrical building, go prone and look at the dial.
Get as close as you can prone, and you will see the needle on the dial moving from left to right, and back across the numbers.
In my game the number was 7781 so I am going to input 7. Just hit it with the crowbar when the needle is on top of the number u want it and it will stop. It is that simple.
Here are the other 3 locations:
The building near quickies-
The same area that you got the crowbar in-
The last location is behind the gas station where the garage for your lab setup is, It is more near the area that goes to the beach
(IMPORTANT: YOU DO NOT HAVE TO DO THESE IN ORDER, AS LONG AS EACH DIAL HAS 1 OF THE DIFFERENT NUMBERS IN THE 4 DIGIT NUMBER YOU HAVE, YOU ARE GOOD)
Once you have your 4 digits in each of the dials, go back to the same room you got the number from and go to the safe.
It will now be open. Go up to it and look at the piece of paper inside and interact to pick it up.
Now you can press tab and look at your number.
(WRITE DOWN THIS NUMBER AND MAKE SURE YOU KNOW WHAT IT IS, YOU NEED THIS FOR THE BOSS FIGHT AND TO BEAT THE EASTER EGG!!)
(Of course not the one in this screenshot, the one in your game)

Now you can move on to the boss fight and the final part of the quest!

Here are all of the compounds and solutions you can have to defeat the radioactive thing and how to make them

Just follow the steps from top to bottom



1,3,5 Tera-Nitra-Phenol
-
Wheel Cleaner + Insect Repellent + Motor Oil= Phenol

Phenol + Drain Opener= Phenolsulfonic Acid

Phenolsulfonic Acid + Detergent= (Solution)




3-Methyl-2,4-Di-Nitrobenzene
-
Paint + Drain Opener + Detergent= Methylbenzene

Methylbenzene + Vinegar + Baking Soda + Detergent= Dinitro

Dinitro + Racing fuel= (Solution)




3,4-Di-Nitroxy-Methyl-Propane
-
Racing Fuel + Quarters= Formaldehyde

Vodka + Pennies= Acetaldehyde

Acetaldehyde + Formaldehyde + Detergent= Aldehyde Sludge

Aldehyde Sludge + Nail Polish Remover= (Solution)




Octahydro-2,5-nitro-3,4,7-Para-Zokine
-
Racing Fuel + Quarters= Formaldehyde

Formladehyde + Glass Cleaner= Hexamine

Hexamine + Plant food + Vinegar + Detergent= (Solution)

I made this guide because on the steam guides area, I did not see anything for this map. Especially for the main Easter Egg Quest. I really hope that this will help players that are wanting to beat this map for either the first time or just needs help when replaying.

Along with this guide, I am making a Beast from Beyond guide as well since there isn't one on steam. So if you need help with that, I will be making it soon. I still haven't finished the boss fight portion of this but I am glad if the other sections have helped you.