So, you're a new level 80 priest, and you've decided to go disc. Or you're building a offset for your guild's Lich King attempt. Either way, you're lost in a sea of bubbles and need some help. Well, don't fret, I'm here to explain the basics.
Healing, as we all well know, is not a rotation or a priority. It's a set of tools and successful healing depends on knowing what your tools are, when to use them, and gearing to increase these strengths. First off, what are disc's tools? When should you use them? And where the heck do all these talent points go?
Spec-wise, you've got a few choices. Wow Popular's Disc Build List is a nice place to start. The top two disc builds are a bit different. This one is what I think of as the standard disc spec, and this one makes your Renews a bit more powerful at the expense of your Greater Heal cast times.
Glyph wise, your standard majors are PW:S, Flash Heal, and Penance. If you're using Renew over Flash Heal (I know some healers do, though I'm not sure why) you might want to switch in Renew for the Flash Heal glyph. Minors are Shadowfiend, Levitate, Fading, and Shadow Protection. You've got a bit more flexibility in the minors, but I do recommend Levitate and Shadowfiend.
Power Word:Shield. Bubbles. If you're raid healing, this spell is your best friend ever. If you're tank healing, this is still your best friend ever. Bubble to prevent or decrease incoming damage, to buy time to get someone back up to full, or just cause you like putting dwarves in bubbles. For tank healing, bubble to get Borrowed Time, a haste buff to get your next heal (Greater Heal perhaps?) off faster. If you're a click-caster like me, bind this to something really convenient and obvious (like left mouse click) cause you'll be using it more than any other spell.
Penance. This is the other iconic spell of the disc priest. Healing with lasers, what's not to like? This is a great spell for NEED HEALS NOW as well as for tank healing and it's also a damage spell. If there's any sort of aoe damage going on, I'd shield myself first, since this spell can be interrupted which means you won't get all three shots off.
Power Infusion. This is a great throw on a spell dps and watch their damage go up spell. I tend to use it when I hear a call for Heroism. This isn't really a healing tool persay, but you can throw it on yourself for extra oomph when needed.
Pain Suppression. This is a great tank healing tool, just keep an eye on the threat meter. Since this spell decreases threat, it's also a nice "oh crap that dps pulled aggro" button.
Prayer of Mending. Start it on a melee, and just make sure it keeps bouncing. I use a hot meter to keep track of it.
Flash Heal. This is my go-to heal. Really. It's not much good on tanks (unless the tank is undergeared) but it's great for topping off dps. My whack-a-mole style of raid healing is a combo of this and PW:S with the occasional penance.
Renew. I mostly use it for an extra tank cushion. Otherwise I don't use it at all.
Greater Heal. Use with the Borrowed Time buff for tank healing.
Prayer of Healing. Again, this is a spell I use with Borrowed Time, mostly when I'm raid healing in a ten man. I don't use it that much, but I have it on my bars.
As far as the other healing spells go (Divine Hymn, Holy Nova, Binding Heal) I just don't use those much. I have seen people use Shadowfiend in combo with Divine Hymn in heavy aoe damage situations, and I keep meaning to try it. Unfortunately, when I'm in that situation I'm generally too busy going "ACK, MUST SHIELD EVERYONE!" to think about it.
Now, it must be said that healing is not like dps, where there is generally one right way to do things for max dps. Healing is wildly situational, and given the depth and breadth of a priest's toolbox, it can be very confusing at first. But there isn't really a right way to heal, and different people have different styles. If you're doing your job, keeping the raid (or the tank) up, not running oom, and not just sitting back and watching the fight, you're doing okay.
Next time: gemming and enchanting, or what do you mean there's no right gem? And why am I fighting with the caster dps for gear without spirit?