Step 1: Bows and Ragnarok
(This Step differs depending on if you play Solo/Co-Op)
Aquire the Wrath of the Ancients (which is done by feeding all three dragons in the Courtyard near Double Tap, The Church above the fireplace and the Undercroft near the pyramid.)

Build the Upgraded Bows: (If playing Solo, only one bow needs to be built, and in Co-Op only the amount of bows that are equal to players ingame, e.g only two needed if two players, three if three players etc.)

Guide on the Bows: http://callofduty.wikia.com/wiki/Wrath_of_the_Ancients#The_Upgraded_Bows

Build the Ragnarok DG-4:
First Part is found by using the Death Ray and looking in the sky, you will then notice a blue object, use one of the Wunderspheres to grab it
Second Part is found after killing the first Panzer Soldat that spawns, after he dies grab the part.
Third Part can be found at the Rocket Test Site, when the rocket test begins, head to the stairs going up to the Pack a Punch spawn, here you'll see a switch open, interact with it and head back to the teleporter, after waiting about 30 seconds, the panel to the right of the Teleporter will be interactable, interact with it and pick up the part inside the Teleporter.

Step 2: Orbs
Re-Equip the Wrath of the Ancients and shoot the orbs on top of the Teleporter in the Undercroft, if done correctly you will hear a Margwa growl play, go and equip one of the Upgraded Bows.

Step 3: Wisps
After hearing the growl, a sparkling wisp will appear in eight locations

Phone opposite Quick Revive in Spawn
Car Tyre outside Double Tap
Boxes near Buildable Bench in the room above Double Tap
Globe near Samantha's Room
Phone Box in the Power Room, near the door to the Death Ray
Radio right next to Speed Cola in the Church/Command Centre
Clock above the fireplace in the Church/Command Centre
Clock on the Catwalk leading from the Clocktower - Church (near a GobbleGum Machine, hard to describe, easy to locate)

After finding a Wisp, shoot it once with an Upgraded Bow (if you miss you will hear a fail sound)
Repeat this step three more times and you'll hear a different growl, confirming you've completed the step (Do Not End the Round as you will need to stay on the current round to perform the next step)

Step 4 - Time Travel
Head to the Teleporter in the Undercroft, there will be a purple glow on it, get all the players on it and teleport, you will be sent back in time, while hear, grab a giant blue vial in the left corner of the room, and fuses to the right. Then wait for Dr Groph to input the code on the safe (be sure to remember or screenshot the code as it is vital for the next few steps)
After making sure all of the items are grabbed, you will be teleported to the Rocket Test Site, fight the Panzer that spawns to get the Ragnarok Part if not yet aquired.

Step 5 - Clocktower
If you haven't yet started the quest to get the Fire Bow, make sure you grab the Wrath of the Ancients and shoot the wall behind the clock in the Clocktower, it will open revealing the inside of the clock, equip the Ragnaroks and place them down, then pick them back up. Head downstairs and hit the switch, the clock will start moving. Stop the clock at 9:35 (big hand VII, small hand between IX and X). After this, lightning will strike the tower if done correctly. To double-check, make sure there is now an extra orb on the pyramid in the undercroft.

Step 6 - Samantha Says
Head to the Death Ray and insert the fuse on the left side of it, then switch the Death Ray from Destroy to Protect, then head back Downstairs to the Safe by the teleporter and pick up the Keycard and Fuses, head back up to the Death Ray and put the fuses in both of the Beacons. Switch the Death Ray to Destroy, Then head to the Computer near the clocktower or on the Rocket Test Site and begin the game.

The four symbols on the bottom screens will show for a couple of seconds before disappearing. Memorize the symbols' locations. The top screen will then show a symbol and the player must interact with the screen correlated to that symbol. Do this a number of times until the machine shuts down and the player hears an audio cue.

Repeat the game of Memory Simon Says at the terminals at the rocket launch site or outside the Clock Tower depending which you did first. If the player fails the game they can instantly restart the game by interacting with it again.

Note that if the player fails the game at their second terminal, they will have to replay the game at both terminals. Also, in co-op, Hellhounds will infinitely continue to spawn while a player is interacting with the terminals.

Step 7 - Sabotage
If done correctly, the two orbs will now be glowing on the Death Ray, simply hit the Green Button and watch the Rocket carrying Dempsey 1.0 on it crash in the Courtyard, pick up the Vril device next to the crashed Rocket.

Step 8 - Tablet and Keeper Quest
Start the next round and repeat the Wisp step (Step 3)
Now when you teleport, instead interact with the book directly to the right of the teleporter, after this a box near the doorway will open, revealing a Tablet, interact to pick up the Tablet, then wait for the Time Travel to finish.

Now head to the front of the Tomb where the Wrath of the Ancients is found, and interact with it, inserting the Vril Device into it will spawn a Ghostly Keeper, follow the Keeper to one of four locations, when he gets to one, simply stand in the circle and kill Zombies with the bow specified (a glow will appear on the screen indicating which bow to use, if playing Solo only one will be needed)

Keep repeating this as the Keeper goes to four locations (note: when the keeper reaches the location near Double Tap, insert the Tablet and put it in next to the Keeper, which allows continuation of the step.

Eventually after completing all Four Locations, the Keeper will return to the Undercroft and merge with the Pyramid, this will now make the MPD from Moon appear, simply insert the missing vial from the pyramid in and you're ready to go.

Step 8 - Keeper Boss Battle
Make Sure you have Perks, Pack a Punched Weapons, the Bows and the Ragnarok DG-4 ready before attempting the fight.
When ready to battle the Keeper, each player must place down the Ragnaroks on one of the squares to activate Anti-Grav, this will teleport you to the boss room.
Boss Fight:

Phase 1: Phase one involves the Keeper using each of the Bows abilites to attack, the Skulls will attack, you can shoot them to avoid damage, the fire pits can also insta-down a player without Juggernaut, and skeletons will also attack (which are identical to zombies)
When the keeper teleports to the Center, one player must place their Ragnaroks down in the circle, if this is not done, hide behind one of the pillars, as the Keeper uses an attack which can insta-down any player standing in the Lightning.
When the keeper is trapped by the Ragnaroks, he will be open to be damaged, shoot the glowing circle on his chest to damage him, keep repeating this until a Max Ammo appears in the middle, this starts Phase 2.

Phase 2: Phase 2 is simple, simply kill all of the Panzer Soldats that spawn in, the first panzer killed will drop a max ammo, and a second will drop after the last Panzer dies.
Once the Panzers have been dealt with, this will start Phase 3.

Phase 3: Phase 3 is identical to Phase 1, except hellhounds will spawn in as well as atleast one Panzer. Simply keep repeating the damage keeper step until Phase 4 starts

Phase 4: Phase 4 is identical to Phase 2.

Phase 5: Phase 5 is the last Phase and is identical to Phases 1 and 3, once the Keeper has been defeated. You've completed the step.

Step 9 - Preventing Future Events
The final step involves picking up the Summoning Key from the Pyramid once you've defeated the boss, once done, take the key up to the Computer near the Clocktower, and watch the fireworks, after this a Cutscene will play and the Easter Egg and Achievement are Awarded.