I play Chogath as a tanky caster. Early game he is mainly farming. Mid game u can just take out huge chunks of the opponents health down while trying to get maximum feast stacks, and in late game his CCs are deadly and ur unnatural size can cover and protect ur carries.
This is my first guide, so it wont be very detailed, but it covers the basics.

Chogath is best at top. At early game his mana cost on his Q is just humangus, which is y i dont use it that much often to farm, just last hit minion with ur auto attack. after lvl 6 dont be afraid of eating minions because u do want the size early and in late game u dont get the chance of it. start using ur skills to farm around lvl 8 because thats when the mp cost is affordable. In mid game u will get the Ap items so thats when u can harrass before the teamfights starts and make caries run before the fight can start. The death cap and rylais can achieve that easily. late game u are very tanky if u have 6 full stacks, but if u want u can switch the void staff for a warmog if u want to. Late game isnt wat chogath shines at but u can go in the fight and start popping everyone up.

I go boots and pots at first, since i am soloing top as chogath. This will make it really easy for escaping jungle ganks early off if their jungler does not choose to get secondary boots early, and u can really farm up as chogath which is wat u really want. Then its the standard ap/tanky build, which is Rod of ages, death cap and rylais. I like to get the giants belt first because the team fights starts around that time. Then i go for the extrea magic pen on void staff and then i chose zhonya's glass over abyssal scypter because first the extra armor is really nessacery since its easy for the ad carry to focus u since ur so huge(i hope) second the active effect its just so useful in any ways. u can go in a fight and pop it up, or knock em up and pop it to make time for the team to arrive

For runes i take magic pen red for the mid game high burst, flat health yellow for the crazy max hp and CDR blue for spamming ur feast and other skills that are really critical in team fights.

For masteries i take 9/21/0, taking flat ap in offense, tanky and CDR in defense. This will make u sustain alot in team fights with that AOE damage reduce and also taking down squishy's health chunk by chunk.

Nothing really special here, i grab my Q first but dont use it that much unless nessacery since it do takes up 1/3 of ur mana.then i do not like to max it and go for a few lvls in my shout(W). then E is very good in farming because it can hit every minion behind the one ur trying to hit. And ofcourse ult at wat ever lvl u can.

summoner spells i take ignite and flash as a standard mage. other than the knock up, once chogath gets in a fight he cant get out of it too fast so i take flash. if ur playing a really competitive game u can grab teleport instead of ignite because chogath can easily steal baron or dragon with that TP/feast if the other team dont pay attention and tp can just initiate a fight with u poping up right in the middle of them.