Well! This is part two... getting into the real nitty-gritty... the actual in-dungeon method. If you haven't checked it out, you should see Part 1 first.
So... your dungeon group is put together. Congratulations! Now stop hyperventilating... Relax and take a deep breath. Take a look at your group makeup, and what dungeon you're in. Recognize what your party members can do CC-wise, and if you're unsure ask. If it's a pug, don't be ashamed or afraid to say that you aren't familiar with the instance you're running. (Or that you've never tanked it before, at the very least). You never know what kind of group you're going to come across. If someone in the group says they've done the place before, then don't be afraid to have them mark things for you so you can learn. Don't be afraid to ask for help. Don't be afraid to use your cooldowns.. EVEN ON TRASH. Some trash hits as hard as some of the lighter bosses. You're going to take a beating. If you feel threatened start using things... just don't blow them all at once.
Now then. There's two ways to start a pull. You can initiate it, and have CC cast right as you're running in, or have the CC cast before you run in, and pull the mobs off of them. Personally I prefer the 2nd way. The mobs are coming towards you while your enrage is... giving you rage. Mobs heading towards whoever cast the first CC, when they're in range hit thrash/swipe. You should have your kill order setup, and marked with a skull (or whatever mark your group has agreed on to die first).
Now, I won't give you a specific set of things to do to keep aggro on a group of mobs, because it really depends on a LOT of different factors. Biggest thing I can tell you is to keep thrash and swipe on cooldown if there's more than two mobs. If everyone is focusing on your kill order, that should generally be enough, but you need to pay attention to everything. Watch your healer's mana bar, your party's health bar, your healer's mana bar, your target's health bar, omen, your healer's mana bar, the CC'd targets in the back, any patrolling mobs that might be coming too close to you guys, your healer's mana bar... starting to get the picture yet?
When I tank, my eyes are darting all over the damn screen. On trash i'm literally only looking at the actual trash mobs like 1/5 of the time because there is just so much everything else I need to keep an eye on. It takes a LOT of practice to get good at it. I can tell you right now, you WILL FAIL at some point. Don't take it too hard. I still screw up at some point. I know the DPS still screw up. Sometimes extra pulls are made, sometimes you may get too close to a CC'd mob with your swipe/thrash.. Don't panic. Panicking is the worst thing you can do, and can easily spell death for your group.
When your group wipes, take a second to take stock of what happened. What could you have done better? Did a mob get past you somehow? Did you position your combat mobs too close to your CC'd mobs? Was the kill order followed? Did a patrol catch you offguard? All of these things can, will, and DO happen. I have done them all... your guild's main tank has done them all... the main tank of Ensidia has done them all. /shrug. Nobody's perfect, and to expect such from yourself is only going to set yourself up for failure.
Now then... how do you keep aggro on packs of mobs? Thrash and Swipe are going to be your main abilities, but as they both have 6 second cooldowns, obviously they're not the only buttons you'll be hitting. I generally thrash, swipe, and mangle my main target, then start targetting my other targets and hit them with lacerates, hitting thrash and swipe as they come off cooldown, or occasionally mauling (you have that glyphed, right?) if you find yourself near the top end of your rage bar (as a rule, it's usually safe at 60 rage or more). Let the DPS do their work. Tab through your targets and keep an eye on your threat relative to theirs. Watch for errant spell-casts. Use your interrupts whenever you see something to interrupt. Don't be afraid to use your cooldowns. [b]ALWAYS keep an eye on your healer[/b]. Watch your healer's mana bar like a hawk in between pulls, and then...
Congratulations! You now have a big ugly boss staring you in the face. Do you know the mechanics of this boss fight? If not, feel free to ask your guildies/group members if there's anything specific you need to know. Unless your group's committed to going in blind. Make sure your group has completely mana'ed and healed up... make sure they're *ready* (yes there is a difference... sometimes people go afk for a split second, or will alt-tab for something briefly)... and then make the pull. Use your enrage to generate some opening rage, and hit it with your Faerie fire.
Now from the pull the first thing you need to worry about is positioning the boss. It's a 100% safe bet to put the boss's back to your party members. Unless there are obstacles that prevent that, any melee in your group will thank you for it. If you're unfamiliar with the mechanics of a fight, or if you know there is are knockbacks involved, putting your large butt against a wall is also a good idea. Now the problem with putting your back against a wall is that it will screw up your visibility immensely. There's a large joke running around that all tanks ever see is boss crotch. Rotate your camera around to your side so you can keep an eye on the party. That way any adds that spring up can be controlled quickly.
Once your boss is in melee range, and while your positioning him, it's time to start building threat... I mean, after all... that's your job as a tank, right? You absorb hits, and you build threat to make sure no one ELSE has to eat any of hits. Your FF will give a fair amount of opener threat. Now your highest threat generating ability, as well as your hardest hitting ability, is going to be Mangle. Mangle should be used as soon as it comes off of cooldown, EVERY time it comes off of cooldown. Watch for your berserk proc'd mangles as well... they take priority over everything else. The majority of your button presses are going to be Lacerate. You always want to get lacerate up to 3 stacks, and then Pulverize. Maul can be thrown in as your rage gets higher (again, 60 rage or higher it'll be safe to maul). I personally don't find the bleeds from lacerate hitting hard enough to be worth leaving stacked while Pulverize's buff runs lower. I could be wrong about this, but it's how [i]I, personally[/i] do it.
With all that, this is what your combat priority should be
Faerie Fire (to keep/refresh the debuff) > Demoralizing Roar (to keep/refresh the debuff) > Mangle > Maul (over 60 rage. It's off the GCD so fit it in anywhere really) > Lacerate x3 > Pulverize > Faerie Fire (if the buff isn't in need of refreshing/you're out of rage for some reason)
It's worth noting that you *shouldn't* run out of rage, but if you slip up and maul at the wrong time, you can definitely find yourself with a free global cooldown. Now, as for your defensive cooldowns. You need to learn what your healer can and cannot handle. If you see anything going on, and your health bar is dropping to an uncomfortable level, pop a cooldown. Depending on what's "uncomfortable" for you, should really depend on what cooldown you use. Barkskin is a short cooldown (And it can be used while you're stunned too), so it's a safe bet to use that one first. If, for some reason, you end up REALLY on low health, use another one (frenzied regen or survival instincts)... The worst thing you can do is panic and blow all 3 cooldowns at once. Sure, you'll take very little damage for a period of time, and your heals will pop you up much higher, quicker, but if the shit hits the fan again within 3 minutes, then you're boned.
Have you noticed a recurring theme here? A lot of what's involved in successfully tanking is keeping your head straight, and not spazzing out. Sure, some people can boil it down to a bunch of math and formulas and stat weights and priorities and such... and to a point those are ALL important, but a tanking is a lot more seat-of-your-pants style than DPSing is. Tanks have to be proactive AND reactive, and legitimately, the entire group depends on your ability to perform your job well. Don't expect to be an amazing tank right off the bat, but take the small successes with the failures, and find out what you do well, and where you can improve. Ask your party members or guildies for advice... find a good bear that you know and ask them for tips on what might help with certain encounters or trash pulls that give you headaches.
Feel free to leave me any comments or questions.