Start Instance Training Before Level 20
Priest is a class-type which is in high demand on World of Warcraft right from the beginning because of its ability to heal others and therefore keep party members alive during challenging battles, and its ability to resurrect the dead when all else fails. In general, Holy-spec'd priests are in high demand within guilds and Instance parties compared to Discipline or Shadow-spec'd ones, but the downside is that Holy-spec'd priests advance through the levels more slowly when playing Solo because Holy focus is on healing, not dealing damage.
In order to stand out as a quality Dungeon/Instance priest, you will need to become a solid healer who doesn't waste mana through over-healing and knows how to keep "threat" ratings to a minimum during battle. It is recommended that you practice these healing and threat-management skills in Dungeon/Instance Pick-up Groups (PuGs) regularly as soon as you've reached level 13 or so, as practice is the main way you'll learn and improve.
By the time you reach level 40, Instance party members expect you have a decent grasp on your class's role within a team, and thus will be less forgiving of errors or lack of preparation. It is at this point that a Shadow priest may find themselves being hasselled by partymates: "Why are you in Shadowform?", "Your job is to keep us alive, not deal damage!", "Stop generating so much threat/hate, I can't get these guys off of you!"
By the time you reach level 55, quality guilds who have members going into Instances on a regular basis may actively blacklist you from their parties, raids or the guild itself if you're not already beginning to show a solid understanding of your healer role. Many guilds are regularly on the lookout for Holy-spec'd priests with instance healing experience, but may deny the recruitment of Shadow priests who have focussed on PvP or dealing damage within solo fights but haven't practiced healing very much.
Group Strategy Differs From Solo Strategy
Playing your toon in an Instance group requires a different strategy than you've become used to while playing solo, no matter what class you've chosen. Different classes have different strengths and weaknesses within a grouped situation, and as your levels increase and you move closer to instances like Blackrock Depths (Level 52+) and into the rest of the "End Game training" dungeons such as Blackrock Spire, Dire Maul, Stratholme and Scholomance, you knowing your class role becomes more of a necessity if you hope to team up with others who know what they're doing.
If you're okay with teaming up with Mages and Warlocks who want to be the party's Main Tank (MT), Warriors who charge in to every fight and "aggro" far more NPC opponents than is truly necessary, and Priests who aren't interested in healing, then don't worry about this or any other Instance Class guide that's posted here - but don't be surprised to find that (especially at level 50+) it's hard to ever actually get THROUGH an Instance in its entirety as a direct result.
Preparing Your Priest Toon For An Instance Run
Before heading into an Instance, it is the individual responsibility of each party member to ensure their toon is in tip-top condition and well-stocked with various consumables. As a Priest, this includes:
Repairing your equipment
Using +healing equipment (as opposed to +shadow or +int or +spi being the focus)
Bringing a stack or two of HP and Mana potions for emergencies
Restocking your food supplies and other materials needed for healing and buffing
If you get involved with Instance parties before level 20, it can reasonably be expected that by Level 40, partymates will be able to rely on you for taking enough responsibility to be properly stocked before entering an Instance. Players over level 40 who join dungeon parties without adequate preparation can expect to be alienated in the future by players who ARE prepared. Other players aren't going to be interested in sharing their consumables and a Priest who "forgets" to bring water and healing mats, for example, takes a severe nose-dive in terms of being a useful contributor to the party.
Common Priest Instance Duties
As a Priest in an instance party, your duties tend to be as follows:
Main Healer (MH), keeping the MT alive as a priority and healing others in the party when able
Managing threat-generation so as not to pull aggro off of the MT or OT
Resurrection of dead partymates
Group buff contributor
Mana pool monitoring (to prevent waste through overhealing)
Dealing Damage, Sharing Healing / Resurrection
As a Priest, there are three major ways you'll cause problems within your Instance party:
Neglecting healing responsibilities in favor of dealing damage
Running out of Mana during average fights due to overhealing/overzeal
Neglecting to speak up when mana pool is low and needs recharging
Hogging resurrection duty when others have a lower-cost option for rezzing
"Letting" partymates die
In short, when it comes to Instances, quality Priests are Healers and buffers, not offensive DPS-machines. Since it is your job to heal the party members who are taking the beats (tanking) while the other classes provide the DPS, for the most part you don't even need to know what direction you're heading or what the boss you're healing your party against looks like.
Beware! Priests Must Be Thick-Skinned
As a final note in part I, I think it's important to note that playing a Priest takes more patience than with any other class. The expectations that other players have of you to keep them alive is enormous, even when they're playing recklessly and ignoring your warnings about your low mana pool. When you're not already actively healing a dungeon party (and sometimes when you are!), you'll regularly be invited into parties by complete strangers, and at times, some of the invitees can be extremely persistent and rude.
As a Priest, you have my full and express permission to do any or all the following:
Declining party invite requests that come from complete strangers without any type of communication confirming your true interest in joining
Adding persistent and/or rude players to your /ignore list
Refusing to join another party with a player who verbally abuses you for your healing skills, practices or ability to keep reckless players alive
Only getting into dungeon parties when you truly feel like going into a specific dungeon
But wait, there's more! As a bonus to this offer, I also give you full and express permission to not feel guilty for having done any of these things.
As a Priest with solid healing experience, the World of Warcraft is your oyster and you'll be in extremely high demand for parties, guilds, raids and the likes. The fact that other players "need a healer" for their group doesn't need to matter to you, especially if they're complete strangers. If they "need a healer" so badly, why aren't they rolling one themselves??!
Stay Tuned For Part II!
This article is Part I of a short series of posts on the topic of effectively learning how to play a Priest toon in a low-to-medium-level Instance or Raid party. Part II will delve more deeply into the bullet-points mentioned in this post, so if you're not sure what something here means, be sure to check back soon for Part II.