Hello! My IGN is Main Got Nerfed and I'm a platinum tier player. I will be doing my best to show you how to properly play Cho'gath and be the ultimate tank.

Why Cho'Gath you may ask? Cho has virtually unlimited scaling and can reach ridiculous amounts of health all the while making his true damage ultimate even stronger.

Grasp of the Undying is a self-explanatory rune. After 4 seconds in combat, your next AA will deal 4% of your max hp as magic damage and heal for 2% of your max hp AND grant 5 permanent extra health. This synergies very well with our massive health pool! The other Runes are for supports or tanks with good engage, so don't even bother with those.

Demolish This is a great rune considering how important taking turret plating is now.

I take Conditioning next to boost the late game scaling we have. While the other runes are viable, Conditioning is simply the best. I build straight tank on Cho'Gath so this rune is a dream come true

Precision secondary is a natural choice known for just being 'good'. Take Triumph and Legend: Tenacity for overall good runes.

Pros

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late game scaling
Strong mid game
Has good, long-range CC
High TRUE DAMAGE ultimate- and it's so satifying too
Easy Farming

Cons

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Easily Kited
Has some bad matchups
You rely on a competent team to deal damage in teamfights

Why Warmog's Armor?
I usually rush Warmog's Armor* because Cho'gath will reach 3k hp with a level 2 ultimate and the completed item (with 6 Feast stacks) which means you can heal back your health after trades. Ultimate lane sustain. Also, the raw health increases the damage on your ultimate and overall survivability.
*If you are having a difficult lane matchup, I would finish boots first (Like Plated Steelcaps against hard AD match, or Mercury's Treads if a hard ap match)

Why Gargoyle Stoneplate?
I generally get Gargoyle Stoneplate right after because of the great resistances it grants. 60 armor and magic resistance. These resistances greatly compliment our large health pool.

When should I buy Turbo Chemtank?
Simply put, you should get this item if your team lacks good engage. Turbo Chemtank allows for a quick initiation and prevents you from being kited as easily. I would get this if the enemy has slippery carries like Ezreal.

Most of the time I wait in a bush until my lane opponent shows up so I can land a cheesy Rupture because your opponent can't see the hitbox if you're in a bush! Then I immediately focus on farming. Proc Grasp of the Undying if you can. Once you get your E ( Vorpal Spikes), you might want to look for a trade, but only if your opponent is still level 1. Always back out after your Vorpal Spikes wears off. Then go back to focusing on the farm. If you're up against a tank, look for more trades than normal. If you're up against a fighter (especially if they have ignite) you should avoid most trades and just farm under tower. The general idea is to mainly farm until level 6. Cho'Gath gets significantly more kill pressure with his ultimate up.

If you know you cannot kill your opponent when you hit 6, just feast on a minion. Do not waste time. You want to get to 5 or 6 stacks by the time you are level 11 so that you can unluck Warmog's Heart passive. Laning should be a breeze after that.

The blue areas are where you should put control wards if you are on the blue (bottom) side. Naturally, the Red spot is where you should put a control ward if you are on the red (top) side.
The yellow places are where you should be regularly warding with your normal stealth wards.

The orange wards are where you should ward before the game begins. You want to ward the enemy buff around 20 seconds before they spawn. This way you will be able to see when the enemy jungler is coming topside for a gank!

To make farming easy, Rupture the caster minions then auto each one of them once. If you're in a hurry, you can use your W to finish them off, but it's quite expensive at 70 mana. If you want to spend less mana clearing the way, use your E on the caster minions. Particularly, use 1 empowered AA on each caster while hitting the other casters with the spikes. After this, each caster minion should only be 1 or 2 auto attacks away from being killed.

TIP: Do not be afraid to use your abilities to farm properly. CS is very important and it is worth using the mana. You should have Doran's Ring to help regen your mana back anyways. If you know you are going to recall soon, use all your abilities to ensure you get every minion.

Cho'Gath can gain a maximum of 6 stacks from eating minions and non-epic monsters. There is no limit to the number of champions or epic monsters you can consume- meaning there is no cap to stacks when gained this way.

If you eat a champion for your first stack, you can still eat 6 minions for stacks for a total of 7 stacks. The order in which you obtain the stacks does not matter.
Always request to feast on drakes and baron. Not only does your feast deal more damage than smite, you will gain an additional stack! Let your jungler know ahead of time because he may be surprised OR you can coordinate with your jungler to smite extra early to get the health of the monster low enough to feast.

Your job in a team fight is to protect your carries and/or kill a high priority target with your ultimate. When I say protect your carries, I mean it. You should be stepping into Thresh hooks so your ADC doesn't get hit. You should be soaking up all enemy skill shots for your team. And when the fight happens, you should use your CC to peel for your carries. Hold on to your Feral Scream until an assassin shows up and he/she will become useless. Use Rupture to knock up any enemies charging to your backline. If your carries are safe, then you can use your Rupture to hit the enemy backlines for a good engage. I think you get the idea by now.

Cho'Gath is a very strong tank that can easily surpass 5k health. With the right item choices, you will feel near invincible, but remember to group with your team to actually win the team fights. Use your silence on mages/assassins during a team fight, use your Rupture to peel for your carries and/or knock up enemy carries and use your feast to execute anyone near you. A stack is a stack, so use it whenever you can get the kill. Go out there and have some fun!

I would like to thank jhoijhoi for making this guide possible. Her coding template taught me everything I need to know for this guide!
If you have any questions or comments, please let me know in the discussion section below.