As I said in the description, this is a guide on how to find Jakito and unleash the foul beast into the world! Before we start, do NOT use/activate cheats to do this. Jakito will despawn and you will not be able to unleash the monster (for the better i guess).

So the first thing you need to do is open up 5-2 and go up the tower where you came from. This is probably the most difficult part of the Easter Egg because it'll take time to scale up the tower.

What I recommend for the first part of the tower is: Do a slam storage (guide to slam storage here), and then shoot a nuke under you (shotgun core + malicious rail cannon. remember that you have to be fast when shooting the core or else you won't be boosted far enough to the top). Once you've reached the top of the first tower, simply do another slam storage and then ride a rocket to the top. It is recommended to finish 5-3 first before doing this as the rocket makes the trip easier.

Edit: I made a video showing how to do it here

Once you have successfully climbed to the top of the tower, you'll see a weird-looking thing inside of a cage.

It calls itself Jakito, and tells you to bring him a sacrifice so it can escape the cage.
(Be warned, you don't really need to get to the top of the tower first before getting the sacrifice, but I just included it here if you feel inclined to.)

After that, just continue doing 5-2 like you would normally would until the boat emerges from the sea. You do not need to get the red skull to progress as you can just slam storage your way to the boat using the lamp post next to the blue skull place.
(The lamp post mentioned is in the image.)

Once you get on the boat, get up to one of the chiney-looking things (i don't know the names), specifically the left most.

Going inside the chimney thing, you'll see a small hamster in a nest of some kind. This'll be our sacrifice. Pick him up.

IMPORTANT: Don't forget to pick up the health orb at the back of the boat as it can save you if you got hurt while on your way to the boat.
(The orb would be somewhere in the image above)

After you do all of that, head back to the tower at the beginning and do the same steps you did last time. Slam storage + nuke for the first half of the tower and then slam storage + rocket ride for the second half.

With florp in hand, place it on the pedestal in front of Jakito's cage and it'll explode (not the big kind) and the cage will disappear. Jakito will then thank you and presumably disappear as well.

When you look to your right, near the watch tower in the distance, a large version of Jakito will emerge from the sea and proceed to crash your game. Be warned if you're running Ultrakill on a weak pc or laptop, the crash might slow down your computer for a bit, although I didn't experience any slow down on my computer when I did this so I doubt you will as well.

And with that, you've successfully doomed the world of Ultrakill by letting Jakito out of its cage.
Thanks to HerbMessiah for the Slam Storage Guide.
And myself for the Tower Climbing Video (shameless plug).
I hope you found this guide helpful!

Edit: Changed all "hampters" to "florps" (too lazy to change guide picture).