T(h)ank God (for) Rammus
Many people underestimate how useful Rammus is in a pure tank role at bottom lane. Although he scales well with funds, the basic tanking is done with his W and with levels rather than relying solely on gear. This allows him to lane with a carry, letting them focus on the last hits, while he assists primarily by harassing with his powerball and taunting for the carry to finish off anybody who stays in lane too long.
MasteriesThese masteries are, I think, pretty straightforward. Awareness is imperative on almost every champ to make sure you keep up in levels, and particularly when you're talking about a duo lane champ that is primarily worried about getting levels instead of farm.
The one point in Improved Exhaust is optional - if you find you'd rather, you could put that anywhere in the Defensive tree, or put it into the utility tree for 15% longer buff duration. I generally let my teammates get blue/red, so I don't bother, and if I'm getting ignite rather than exhaust then the move speed from dodging is a better choice.
Note that the masteries and runes are a WIP for me, as I am not yet level 30, and am writing this from the perspective of normal games, where you don't always have a team that can handle jungle Rammus (and to be honest, in competitive games I suspect you should be playing a different champion, like Shen).
RunesRunes are pretty simple - Flat Armor Quints/Seals (for early game damage and tank), Magic Resist/lvl glyphs (since you're less in need of the MR in the early game, but this way you're stacking MR as your opponents are stacking AP, to reduce their damage a little bit more).
The Marks are really the only questionable decision - I find the flat armor pen is nice for the minion farming that you -do- get the chance to do, but you could select magic pen runes here instead if you'd prefer to see your ultimate and powerball eating through MR, or even pick up secondary armor runes, etc.
Flash is (in the current meta) a near-necessity - it lets you get over walls, creep waves, etc, to make sure your powerball lands. It can also be a useful escape - tower dive in the early game with powerball, taunt/DBC and kill your target, and then flash out before DBC drops and the tower eats your last shreds of HP.
Exhaust is an excellent choice for a tank, who is likely to be in range to drop it on their AD carry, significantly reducing their damage output. It can also be used in conjunction with flash to get in range of a running enemy and slow them down to get you in taunt range if you can't powerball to them in time.
Ignite instead of Exhaust is a good pick if your team doesn't have it already, or if you're anticipating a lot of lifesteal or other healing on their time (Swain, Warwick, Soraka, Fiddlesticks, etc), a well-timed Ignite can really make the difference in a teamfight between dropping "that one champion" to win the fight for your team and getting owned because their carry/support just won't go down.
Ghost instead of Flash is an acceptable pick if you find yourself hampered by raw speed instead of by obstacles, or if you're low level and don't have flash yet.
Fortify is an acceptable second spell if your team also contains a heimerdinger - you can (in theory - this has happened in a few of my games, but not very often) fortify a tower, saving it from immediate destruction, and then heimerdinger's aura can repair it back from the brink. Note that unless your game is very close, this is probably not the best use of your spell slot - if you do get this, make sure to drop improved exhaust for improved fortify.
Passive: Spiked Shell
Rammus gains additional damage as his shell becomes reinforced, converting 25% of his armor into damage.
Rammus's passive means that, against AD-heavy teams, you're going to end up being able to do a little bruising yourself in teamfights. For some Rammus builds, this is pretty awesome - for us, it's just a nice little perk. Don't focus so much on armor that you don't build towards what the enemies are actually doing.
Rammus accelerates in a ball towards his enemies, dealing 100 / 150 / 200 / 250 / 300 (+100% of ability power) magic damage and causing 20 / 25 / 30 / 35 / 40% reduced movement speed to nearby enemies on impact for 3 seconds.
Cost 80 / 90 / 100 / 110 / 120mana
This is a one-point wonder for Tank Rammus - you'll mostly be using it to initiate a teamfight, rolling into their highest DPS carry or most annoying CC support and taunting them with E, then immediately popping your W to protect yourself from their damage. It can also be used in the cleanup portion of a teamfight to catch whichever one of their ranged carries had the sense to run away before the entire team got welped, and can allow you to grab them with taunt long enough for your team to come get the ace.
W: Defensive Ball Curl
Rammus goes into a defensive formation for 6 seconds, increasing his armor and magic resistance by 50 / 75 / 100 / 125 / 150 and returning 22 / 26 / 32 / 38 / 46 +10% of total armor as magic damage to attackers.
This is what makes you able to do a number of silly things - you can powerball into somebody hugging a tower with low health, taunt them, and DBC to eat the tower's damage while you finish them off (or your lanemate does). You can survive standing in the middle of their team while your teammates drop their DPS as they desperately try to kill you because you've taunted their support.
Mostly, however, what makes this great, is that it reduces your reliance on tanking gear, letting you give your lanemate the last hits and therefore the early-game gold.
E: Puncturing Taunt
Rammus taunts the unit into a reckless assault, reducing their armor by 10 / 15 / 20 / 25 / 30 and forcing them to attack Rammus for 1 / 1.5 / 2 / 2.5 / 3 second(s).
Cost 50 / 65 / 80 / 95 / 110mana
Taunt is an important skill, both offensively and defensively. All too often, their AP carry (usually something with crazy burst damage) will see a low-HP Rammus and run in for the gank - at which point, I'll pop taunt on them, suppressing their abilities as I lead their squishy selves right into my tower, promptly letting it eat them alive. I'll usually combo this with DBC and come out the other end due to my extra magic resist.
Offensively, you'll want to follow up your powerball initiation with a taunt and defensive ball curl, usually taunting their strongest damage (as I stated in powerball - see how his abilities are synergizing?)
Rammus creates tremors beneath him, dealing 65 / 130 / 195 (+30% of ability power) magic damage per second to nearby units and structures. Lasts 8 seconds.
This is nice when you're in the heat of the battle - I generally pop it when I'm cuddled up to all their champs right after a powerball. Another -very- important note is that this does damage to structures - combined with defensive ball curl, you can do significant damage to a tower, even while under fire. Just make sure you pull back in time to not get eaten by the tower fire.
Skill OrderAs a general rule, you'll want to put your first point in Powerball - this can be swapped for Puncturing Taunt if you plan on a level 1 teamfight (for example, if you're going to gank their jungler at Blue, or expect to defend your jungler). After that, you'll want to pick up level 1 of the other two skills, and then you want to max out taunt as much as possible, taking defensive ball curl as you see fit to keep your armor and MR bonus up. Against heavy DPS teams, I find myself maxing DBC immediately; against a slower building team or if my team is doing well already, I find the longer taunt duration very useful for letting your carries dump damage onto your target.
Item BuildsAs stated in the mention about items, tanks need to be fairly reactive. You're likely to want a Philosopher's Stone (early health+mana regen, and gold/5 never hurts) and therefore start with a Regrowth Pendant, though in premades where I expect to be able to farm my opponents a bit better, I go with a Ruby Crystal (especially if you're laning with Soraka or Sona, both of whom can add sustain to your raw hp with heals).
Once you've built your Philosopher's Stone (if getting it), pick up your Boots of Speed. 90% of the time, you'll want to build this into Merc Treads - although occasionally you'll want Boots of Swiftness/Mobility (both great for jungle Rammus, but poor for lane Rammus) and if they're AD-heavy and low on CC, you'll want to pick up Ninja Tabi (which, if you get the move speed from dodging mastery, gives you a nice bonus speed most of the time).
What I tend to do at this point is rush a Warmog's - a maxed out Warmog's will give you a significant amount of HP, and DBC will give it a nice shield to eat even more of their DPS.
After you get your warmog's (or possibly after your Giant's Belt, if you find you need to hold off on the Warmog's), pick up a Negatron Cloak or Chain Vest, depending on your opponent's team comp - check for who's doing the most damage on their side and build your first one against that. In most cases, the team will be balanced enough that you'll want to pick up one of these, followed immediately by the next.
I constantly evaluate at this point - do I need a Force of Nature, or would a Thornmail do more good? Are they squishy enough for a Sunfire Cape? Do I die because they pop all their ults on me, and if so, would a Guardian Angel be worthwhile? There's no good way to describe this further; you'll have to play with it for a while and get a feel for what the "right" build for a given game is.
Final CommentsI hope this guide helps some of my fellow newer players. This is definitely a living document, so if you have suggestions, please feel free to leave a comment.
I'm still going to look into putting up some videos; on that note, I would also suggest that if you want to take your game seriously, get LOL Replay and watch both your wins and your losses.