THIS IS MY FIRST GUIDE!!!
Kog'maw is a hypercarry who really hurts during late game the 2nd most powerful AD Carry during late game, Vayne being the most powerful.

When the prophet Malzahar was reborn in Icathia, he was led there by an ominous voice which thereafter anchored itself to his psyche. From within, this voice bestowed upon him terrible purpose, and though Malzahar was no longer tormented by its call, the voice did not cease its unrelenting summons. This baleful beacon's gentle flicker - now fastened to Runeterra - drew forth a putrid beast that ambled across a threshold it did not understand, widening a fissure between spaces which were never meant to meet. There amongst the haunting ruins of Icathia, Kog'Maw manifested in Valoran with unsettling curiosity. The spark which led him to Runeterra teased him still, urging him gently towards Malzahar. It also encouraged him to familiarize himself with his new environment, to the stark horror of everything he encountered on his journey.

The enchanting colors and aromas of Runeterra intoxicated Kog'Maw, and he explored the fruits of the strange world the only way he knew how: by devouring them. At first he sampled only the wild flora and fauna he happened across. As he traversed the parched Tempest Flats, however, he came upon a tribe of nomads. Seemingly unhampered by conventional rules of physics, Kog'Maw consumed every nomad and any obstacles they put in his way, amounting to many times his own mass and volume. The most composed of his victims may have had time to wonder if this was due to the caustic enzymes which stung the ground as they dripped from his gaping mouth, although such musings were abruptly concluded. When his wake of catastrophe reached the Institute of War, an enthusiastic Malzahar greeted him with an enticing prospect: taste the best Runeterra could offer…on the Fields of Justice.

"If that's just hungry, I don't want to see angry."
― Tryndamere, the Barbarian King

Pros:

Strong Late Game

Cannot Be Easily Countered By Tons Of Armor

Can Perform Well In Teamfights

AoE Slow

Good In Last Hitting

Dangerous When Dead

REALLY PAINFUL!!!

Cons:

Squishy

Weak Early Game

Ult Reliability

Mana Hungry

Easily Countered By Silences

Let Me Explain:
You need your CC during level 1 to avoid ambushes
You need your "W" for kiting
You need your "Q" for removing the enemies' armor and magic resist
You need your "R" as Early as possible because it has a long range and deals
(50% of AD x 225% if it's a champion + base damage)= 112.5% of AD to champions plus (180/270/360)
base damage.

Q: Why max out "Q" first before "E"?
A: It can be used for last hitting since it has a longer range than your basic attacks
(without "E")

Blink > IGNITE!!! > "E" > "R" > "Q" > "R" > "W" > SPAM ALL YOUR SKILLS AS LONG AS YOU WANT!!!

EARLY GAME: Farm with Basic attacks and kite with "W".
-DURING LEVEL 3: Hide in brush > use "E" > use "W" > use "Q".
Mid-Game/Late-Game: Just Farm and Gank!!! Join Teamfights as well.

You need peelers during Early/Mid Game.
Spam your skills!!! You may bring a Crystalline Flask if you want.
NEVER EVER RUN IN A 1v1 or a 1v2, IF YOU CAN USE "E" BEFORE DYING, USE IT!!!

Q: I have a lot of killstealers in my team what should I do?
A: I didn't require you the Blade of The Ruined King for nothing.
Q: They are building tons of armor, what should I do?
A: Use your "W"
Q: They are building tons of MR, what should i do?
A: You are an AD Carry