As I hinted in the opening, the first major step you need to prepare yourself for the encounter is to tinker with your glyphs load-out. For majors, make sure you're sporting the Glyph of Holy Wrath (crucial for transitions) along with the Glyph of Divine Protection. Make sure to nag any other paladins in the raid to glyph Holy Wrath as well!
Glyph of Focused Shield is also a useful choice. You'll only be tanking, at most, two things at the same time. So you can enjoy the extra DPS without worrying about hindering your threat spread.
Lastly, I'd recommend (if you get easily distracted, like I often can) making a Power Aura for Burning Wound, the major thing we need to pay attention to in this fight. I have a Power Aura that tracks the duration of my debuff next to my health bar, so I know that when it disappears -- and thus ticks off -- it's time for me to taunt. More about that in a second, though.
Phase 1: Taunting between catnaps
As the subheader implies, this is probably the most boring phase of a generally boring fight, at least for tanks. Your biggest responsibility is managing the Burning Wound debuff and taunting at the appropriate time so you and your co-tank aren't inflicting too much unnecessary damage on the other through inaction.
After the pull, I usually hold the boss until about three stacks. At that point the other tank pulls it off me, and from there on out we trade when the debuff falls off. Rinse and repeat, and repeat, and repeat. Ad nauseam.
Likewise, this phase abounds with stuff you don't want to stand in, but usually you won't be anywhere near it or generally won't have to worry about it. Stand on the lip of the platform right at the lava's edge, so when Sulfuras Smash comes down you won't have to worry at all about dodging it. Ditto the follow-up Lava Wave. Standing in the direct middle of the room has never led to my getting knocked back into it; you should be equally safe.
As for knockbacks, you can thank Hand of Ragnaros for that. After the shove, just run back in. Ragnaros' hitbox is gargantuan, so you don't need to worry about accidentally going out of range and provoking a Magma Blast.
Moreover, I don't think you have to worry about a Magma Trap spawning anywhere near you. If it comes down without 8 yards, however, you'll have to move to avoid the damage of the Magma Trap Eruption.
Transition 1: Stun like you've never stunned before!
When Ragnaros hits 70%, he'll cast Splitting Blow, slam down Sulfuras, and retreat under the lava. Following that, Sons of Flame will spawn and begin to rush toward the hammer.
This is the part where paladins of all specs shine, thanks to our amazing toolbox of stuns. Hammer of Justice gives you a 6-second stun, and a glyphed Holy Wrath an additional 3 seconds. The plan should be to park by your appointed Son, wait for it to appear and begin DPSing. Especially post-nerf, its health will melt away quickly, so don't forget to actually hit it! Once the health of the Son is reduced to 50%, it can't get any slower and will attempt to nigh crawl to Sulfuras in a vain attempt to Supernova. Don't let it!
Part and parcel, don't blow your stuns too early. Wait for it to being moving (there's a second or two of lag time before it is attackable and starts moving) so you don't waste a single second of stuns. Then, once Hammer of Justice is used, position yourself between your Son and the next incoming one, so when you pop Holy Wrath, you'll hopefully hit more than one. Nine seconds of lockdown is more than enough to get a Son to 50% health.
Once all the Sons are dead, Ragnaros will emerge and the next phase will begin.
Phase 2: Keeping you on your feet
On top of the gimmicks from the previous phase, there are some new attacks to avoid or handle. Continue to sit on the lip of the platform while tanking and dodge Lava Waves, along with taunt trading with your co-tank.
On top of that, don't get too comfortable, as Ragnaros throws out some monkey wrenches that demand attention and immediate movement and cooldown usage. For starters, he'll cast Engulfing Flames and burn up a third of the platform, horizontally. Clearly, don't stand in that.
In addition, at set intervals he'll spawn a Molten Seed where everyone was standing. When these go up, you'll want to collapse to a set point. As a result, the raid should be split 50/50 on either side of the platform prior to the seeds' emergence. Because the resulting Molten Inferno does more damage the closer you are to the seed, having seeds in the middle is a sure recipe for a raid wipe.
Once the Molten Inferno occurs (and it goes without saying that this is an excellent time for Divine Guardian), Molten Elementals will spawn from the spent seeds and rush toward the raid. They'll fixate on random players, so you can't tank them; don't stress about that, and instead focus on damage mitigation and AOEing down the adds. Unfortunately, the Molten Elementals' Molten Power buff prevents them from being stunned via Holy Wrath -- alas.
Make sure to also mitigate the explosions with judicious use of Divine Protection and the clicky on Mirror of Broken Images. The less damage you take, the more healing that can be applied to the rest of the raid.
Seeds and friends repeat themselves two more times, though post-nerf we've been able to completely skip the third by DPSing down Ragnaros to 40% before he can cast it and forcing a phase transition.
Transition 2: Finally, some adds
This transition is nearly identical to the first one, except for one major change: Lava Scions. They'll need to be picked up immediately while you still get your normal stunning duties off. At the same time, people need to be cognizant of the Blazing Heat debuff the Scions will be throwing out and making sure that they don't get roasted by it. Fancy footwork and all.
We usually put a tank on each side of Sulfuras, charged with picking up their side's Scion. They hold it for the duration of the transition while doing their stuns. Once the Sons are dealt with, have one tank pick both up and get in position for the last phase.
Phase 3: Keep rolling to victory
This phase is a culmination of the previous two Ragnaros phases, employing his tricks from both along with some new ones.
Sulfuras Smash persists along with Engulfing Flames. No seeds or Magma Traps, thankfully.
As the phase begins, you and your co-tank want to get to a corner of the platform so that you're nowhere near the path that the meteor might be kited. This should primarily be a DPS concern, and if some knuckle-dragger drags his big, flaming rock and asplodes you, feel free to rain blows upon them. There can be no acceptable recourse but that.
Continue doing what you did the previous phases: taunt, nap, taunt, nap. Eventually, someone will wake you and tell you that Ragnaros hit 10% and retreated. Celebrate with the liberal application of loot.
The fruits of your labor
Taunting is hard work! Clearly, we've earned the following pieces:
Immolation Faceguard This comes from the tier token that Ragnaros drops. It's okay, but there are better options in Firelands for survivability.
Pauldrons of Roaring Flames This is a good choice, especially if you're having trouble scoring a tier token. However, the tier shoulders are worth a full .64% more combat table coverage than these, so if you do have the tier shoulders, stick with them.
The Light and How to Swing It shows paladin tanks how to take on the dark times brought by Cataclysm. Try out our 4 tips for upping your combat table coverage, find out how to increase threat without sacrificing survivability, and learn how to manage the latest version of Holy Shield.