This is a simple guide, Nowhere close to the best build for rammus but this is how i play rammus personally, I've mained him ever since i can remember, this build wins me 9/10 rammus games i play since i can and have single handedly turned the fights around with my tanking.
This build is built for early game hp regen building into armor and 40% slow for team fights way more health, then later on mr for when the ap carries start targeting you and finally slow for stopping people from running away when you're raping them.
Of course if cc or ap is a problem early on, build force of nature before warmogs and replace tabi with merc treads.
The second build on my page was a friend of mine who uses a split apart side of this build for less gold heavy so soon into the game, almost the exact same just a bit tweaked.
I take heal from time to time if i really dont trust my team comp and feel i need to be more tanky.
Flash is what i use when i have some faith in my teammates.
Pros / Cons
Good for team fights
Late game you can steamroll anything.
You can troll hardcore if you pick summoner heal, heal on rammus late game annoys the everloving **** out of most teams fighting against you.
You must be a great ganker to pull this off, you need a lot of gold to get the most from this build
If you cannot farm fast enough you'll fall very far behind.
Less gold heavy than the first build
Good for farming with the sunfire cape
Good damage output
Still pretty beefy endgame
Not quite as tanky
Squishy pretty late into the game since you finish randiuns late
Might lose team fights from the late randiuns
The current meta is more ad than ap, it builds ap first and ad last so it's risky
Once you have your omen it's your job to use it properly, in the middle of team fights if you activate it 40% attack speed slow will turn the whole battle in your favor if you hit 2-3 of the enemy team.
Not much else to say besides the basics, target squishies and if a team mate is running away almost dead, lock up the longest ranged poker or closest person chasing them and taunt to save their ***.
Late game rammus is a brutal tank, don't be scared to turret tank for a teammate to take a turret. I can't tell you how many times i grab my akali friend and backdoor a turret in a few seconds because each blast does less than 1/30th of my hp bar.
If an enemy is going under a turret and almost dead go for it! after warmogs you can tank turrets for quite a while, it's worth it most of the time if you can pick up an easy 300 gold and walk way like nothing happened.
Your level 3 ulti also makes it very viable to solo backdoor a turret, but never overstay your welcome, even as a rammus 4v1 at their second or third turret almost always means they chip you down enough to kill you before you get to a safe zone
Creeping / Jungling
I have 1-2 from mid and bot hit wolves for me once, then run over and wait for me at blue, i then attack the golem first and turn on defensive ball curl, i have the two helping me dps the golem down to 40% hp and leave, by the time this happens, my curl will just finish and i can smite the golem to death, i run to the wraiths, wolves, red, wraiths, golem then usually B to buy my philo stone, regular boots and hp pots. The rest is up to you which route you take.
When ganking, i alert the lane I'm going to gank very early in advance, since my powerball lasts quite a long time and can cover a lot of ground try to take the long way so you can come behind the enemy, lock them down and hopefully get the kill.
Red buff is extremely good for rammus, on top of his powerball, redbuff guarantees you get the taunt off and can chase your enemies
Now i know people always say wriggles is gods gift to junglers but honestly for rammus, the gold is better spent elsewhere.
A bit into detail with why i picked what i picked.
Regrowth and hp pot at the start builds into philo which gives me gold the whole game.
Mercury treds: cc and ap is not longer a problem.
Ninja tabi If they are 4/5 ad and have little cc this is really nice
heart of gold: For the extra hp and money.
Wardens mail: For hp regen and armor.
Randiuns omen: Armor, hp regen and that oh so juicy active ability, it's won me more team fights than i can count.
Warmogs: Because 4k health is always nice.
Force of nature: I don't like being squishy against ap and with shurelya's you can get 135 hp per 5 seconds back, that's a major f*** you to the enemy team trying to kill you.
Shurelya's reverie: Health, hp mp regen ability to get in and away from fights like nobodys business with powerball. also with 20% cooldown reduction you can use your ult taunt and defensive ball way more often to get even more kills!
Frozen mallet: For the hp and slow.
This makes me pretty beefy endgame and can shutdown bursters like akali and talon without much effort.
Something to keep in mind.
If the enemy team is heavy ap you might want merc treads instead of ninja tabi and force of nature before warmogs.
If the enemy team is 4/5 ad i highly suggest using these two
Thormail: 100 armor 30% damage sent back at them
Sunfire cape: Hp and damage that works with taunt and thornmail
instead of frozen mallet and force of nature, with rammus's taunt whoever you catch will melt in 3 seconds flat.
Atmogs is also viable if you want to give up frozen mallet for more damage, but i personally find being tanky with that killer slow better suited for rammus.
Quicksilver sash Very situational item but if you're losing to hard cc it might be handy to pick one of these up.
Banshee veil Very situational but if an over fed karthus keeps destroying you with his ult this might be the best option.
Hexdrinker Very situational works like banshee veil but not quite as good, it's quite a bit cheaper.
Wriggles lantern No no no no no no no, don't touch this, this is a waste of gold on rammus, do not ever touch this horrible item, rammus can use that gold so much better than throwing it away like that.
Fighting against rammus.
If you find a rammus laning, you win, simply put rammus cannot lane worth a damn, he sucks don't try it no one is careless enough to walk into your taunt range.
My reasoning to support this. To get the items that rammus needs most he needs cs, in lane 90% of the time you are with a carry, they need cs more than rammus does so you get starved and build everything extremely slow, jungling is the best way to play him.
On top of all that, rammus, early game he is very squishy and susceptible to being poked.
He's super tanky for about 4 seconds, in lane no one will poke you those 4 seconds, they'll wait till you're out of it to poke you.
Tanks that can lane are tankier than rammus early game because they don't rely on a skill to do all the tanking for them.
Rammus also has **** all for harass in lane, powerball can be avoided without much trouble when you see it coming, taunt is almost worthless till the 3 second length, people like voli galio or blitz can harass a lot easier.
Dealing with rammus is a bit tricky if they know what they're doing, your best bet is to just avoid him till your tank is around, keep your distance and poke him from behind minions so he cannot powerball you.
When laning try to stay to the safe side so he cannot pop out and powerball, make sure he will have to go through minions.
If you are within 1 rammus length of him he will taunt you so use his body 2x as a good field to avoid.
In team fights stay spread and let him burn his ulti and curl, once his curl is up he's relatively squishy, if you have any plans of taking him out do so IMMEDIATELY after his curl ends, you know it's ending by the colour change.