It's process time again! We do so love our organized processes. Perhaps you've always wondered how a healer should approach a new encounter and put together a coherent framework of action (doesn't that sound professional, like some business world/World of Warcraft hybrid?). Perhaps not. Actually, probably not. I'm going to tell you how, though.
Healing, in my view, can be distilled to this: matching patterns of incoming damage with patterns of healing. In dungeons, things can be a little random (what with tanks of varying skill, DPS of varying desire to pull extra packs, etc) but in raid encounters, things tend to settle into fairly understandable and predictable patterns.
Consistent tank damage is one pattern of incoming damage. Spiky, inconsistent tank damage is another. Consistent damage to the raid (like a persistent damage aura) is yet another. Bursts of single-target damage to random raid members is even still another. I could go on like this for days, but you get the idea. Damage has a shape, a profile...it affects life bars in different ways.
Those differing profiles or patterns are what make healing dynamic. If damage only had one profile (say, evenly distributed damage across everyone in the raid), it would strip the dynamicism from healing the encounter. Ultraxion is a lot like that and it's not exactly the most thrilling fight to heal (though sometimes it's fun to have a fight where your job is simply to pump out the biggest sustained numbers you can).
So you could enter a raid encounter with little preparation, beyond knowing what you need to avoid standing in, and see how the life bars change and cast an appropriate spell. Even for reactive healing specs, though, that's not a good idea, as you'll always be a step behind. The "step behind" is not that you're casting the spell after the damage happens...other than discipline priests, that's how healing specs work. Where you'll be a step behind is needing to recognize the incoming damage profile and choose the correct spell to counteract it before you can cast. That's a significant delay that adds up over the course of the encounter, leading to less useful healing than you'd otherwise be capable of.
Study The Game Film Between Games
Read about the encounter...the mechanics you'll be seeing, the damage they do and the recommended strategy. Now, the recommended strategy may not be what your raid team does, but it's valuable just to get a general understanding of what types of tasks your DPS and tanks will be carrying out and what they'll be subjected to.
Watch a video, too, if you can. Patterns can often make more sense when you see them play out.
Based on this research, start to build a plan for what spells you're going to want to use and when. When you're going to need to go all out, guns blazing, and when you'll have some respite to perhaps pop mana cooldowns or generally recover a little (or, at least, ease off your most expensive spells). Especially as you do heroic encounters, you'll pretty much need to be casting something at all times...taking time to decide what it is you need to cast will slow you down. You want to have a sense for what you're going to be casting when ahead of time.
The Scientific Method
Healers are nothing if not scientists. So you have your hypothesis about what patterns of casting will see your group through, but hypotheses need to be tested. Every attempt on an encounter is a nice controlled experiment in the lab. Start off by sticking to the plan you crafted beforehand.
Things are going to fall apart the first time, if it's a difficult encounter. That's part of the fun--data collection! So keep a sense for when things seemed to fall apart (generally it'll be memorable as the point in time when you throw your mouse through the drywall) and, after you've thrown your mouse, try to stay clinical. What happened? What went wrong? Did everyone else stick to the plan? If they did and the damage still got out of hand, then perhaps you didn't have the right spells planned for that part or maybe you miscalculated what the damage profile would even look like. Sometimes, the intersection of several mechanics can be hard to predict, a priori, in terms of how they will combine to hurt the raid.
But that's cool, you're still in good shape. Repair up and adjust your strategy a bit. Ask yourself a few questions: Did I seem to be healing the wrong people at times? Did I use spells that were too weak when bigger spells were needed? How much will patching up that drywall cost anyway? Subtly, but crucially, could you have used different spells at earlier times in the fight that might have helped the situation when things fell apart?
That last one, I feel, is easy to overlook. Generally, and understandably, people gloss over the points in the fight that seemed stable as "fine" and examine only the points in the fight where the deaths began to pile up.
Obviously, the failure point probably has most of the important post-mortem information. Failed raid encounters often boil down to certain critical junctures and once you have those sorted, you have the fight down.
However, as the ripples of anger and fear in Alberta can eventually culminate in a stock collapse in London* so too can decisions early in the fight lead to consequences later in the fight.
*Theoretically. It's probably never actually happened.
As an example from my experience as a discipline priest, perhaps you could have cast a bunch of shields while things still felt stable in preparation for the period of time that even casting all your fast, expensive spells was not sufficient. Perhaps if you had focused on keeping the tank higher in health at a different point, she wouldn't have used a defensive cooldown and she'd have had that cooldown available at a more critical juncture. Perhaps if you had matched the right spells to the situation earlier in the fight, you'd have more mana to blow at a big and important healing moment.
Figure out not only what went wrong at the time when things went wrong, but also what decisions earlier in the decision tree might have averted the crisis. Each encounter is a bunch of new data...use it to refine your hypothesis.
Insert Cooldown Here
I think it's important to first figure out your general plan of action, which is why I haven't mentioned cooldowns to this point. Sometimes it'll be obvious just reading about the encounter when cooldowns will be important and sometimes the raid leader will assign your cooldowns to certain places in the fight. That's fine.
But it's not always clear when cooldowns should be used. Not every fight has an "unusually massive blast of damage now" moment and each raid group does an encounter slightly differently (or a lot differently). In those cases, you'll have to figure out when a cooldown will do the most good by seeing where your raid team has the most trouble and mentally marking those times as the points at which to insert your cooldowns into the fight.
But remember: when in doubt, use the cooldown at the earliest moment that it'll be useful. Cooldowns are powerful abilities that Blizzard doesn't want you to be able to use too often. The less you use your powerful abilities, the longer the cooldown time you are adding to it. After seeing the fight enough times, you may gladly trade using it less for clutch usage to get you through moments you couldn't survive otherwise.
Talk To Other People (Your Fellow Healers Are People, Too)
"Talk to them" is not necessarily literal. I'm also including reading the thoughts of other people on the internet, even if you don't interact back. So many people are doing all of these same encounters that you'll see a lot of opinions. As per Minstrel's Law of Everything: 90-95% of everything (stand-up comedy, music, opinion) is crap. Especially when it comes to the Internet. But that other 5-10% can be pure gold.* If you spend time reading over what other people say, you may come away with one or two solid ideas you didn't think of to make the encounter easier.
*My blog: 90-95% crap, 5-10% gold. This is intentional; I encourage you to figure out what is what, it builds character.
Your healers are particularly worth studying as specimens, as they get to experience the same situations as you, unlike healers in other raid groups. See what they're thinking, what they're feeling, what cultural backgrounds are prejudicing their healing.* It's not impossible that a little communication can cause a revelation that will change how you address a pattern of incoming damage.
*"I'd expect you to feel entitled to our feral druid's Innervate. Everything's just come so easy for you ever since daddy got you a job as a vice president, hasn't it, Spoiled Bobby?"
The Main Point
The main point here is that your job as a healer is to identify the shape of incoming damage over time and use the right spells to match that pattern. You should have an approximate idea of what that pattern will look like in your head going in, so that you have a testable plan. Being approximate, however, means that there will be certain crucial inaccuracies, so you'll use the wipes to analyze and refine both the picture of the damage over time that you had going in and your plan to address it.
Have one part of your mind dedicated to being an impartial observer that doesn't care if you win or lose, it just wants to know what happened. Leave the fiery passions to the DPS...your job is to stay analytical at all times. The more you observe and understand, the stronger the refinements to your plan.
The most important piece of gear for you as a healer is your brain. (Seriously, guys, I just came up with that. I'm always coming up with stuff like that off the top of my head at parties.)