ahah thought it was just me! when i first started healing (after playing rogue for years) i would fucking panic like a little girl!
- i would use the +10crit chakra in dungeons (until you gear up anyway, at which point you'll have it all figured out anywho)
- try to use 'heal' exclusively, don't be afraid to use other spells when necessary tho
- don't panic when poeple go down half their health all of a sudden. if it's in the fight mechanics, you will have enough time to heal it. if it ain't, it's their fucking fault not yours ;p gl
The most important thing about Holy is knowing a lot about the incoming damage. Otherwise you'll find yourself running out of mana spamming in places you don't need to be. So if you're comfortable with the encounters then your healing and mana management will improve.
As has been said before - in 5man instances you'll use Chakra: Serenity almost exclusively (I only used Sanctuary in Halls of Origination and sometimes on High Prophet Barim in Lost City of the Tol'Vir).
Don't be afraid to put up a Lightwell before boss fights and tell people to use it. Even if only you yourself use it for self heals it is still free heals (as you will have regenerated the mana expense before the fight has really started).
Use PoM in fights with group damage, it's a great spell.
Don't be afraid to use "Heal", it's your most important healing spell until you hit the Zandalari heroics (imo). Heal, Greater Heal and Holy Word Serenity are your stock heals. Surge of Light will make your life so much easier. Use Holy Word: Serenity on the tank whenever possible (due to increased crit chance -> Inspiration proc) and always keep up Renew on the tank (again a free heal).
If you need to increase throughput use Flash Heal or Binding Heal up to 2 times to proc Serendipity then continue with Greater Heals.
Avoid AoE healing/group heals when possible - they are expensive and lead to overhealing. Often you will have enough time to heal everyone up with single target heals before the next ability hits with unavoidable group damage.
Don't top off dps players (especially not when using group heals) - you'll just snipe your own Echo of Light.
Use PW: Shield only for last second saves of careless dps players (that pulled aggro) or for Body and Soul. It is too expensive to be used as a stock spell for Holy.
Divine Hymn is very strong and mana efficient, know when to use it (and try to heal up the tank before using it^^)
You will probably have to use Hymn of Hope in some fights (I did a lot during regular cata dungeons and sometimes in 346 hcs) - use knowledge of the fight to know when you can use it and cast PoM and Renews before doing so.
Don't hold on to your Guardian Spirit too dearly - it's ok to use it just for the healing increase (especially if you are on the verge of going oom). As you won't talk to the tank about cd usage (unless you are in a guild group) chances are he will use his own cds in a critical situation (without noticing that you just did cast GS on him) anyways.
If the boss is on 10% and you are oom just step into the fire. You will be much more useful as a Spirit of Redemption than you would have been alive.
Mythical Mana Potions are pretty cheap now on my realm.
Recount is your friend - knowing which spells did lead to overhealing can be really helpful when you are struggling with mana issues.
Dispel > healing - mostly a good idea.
If the tank thinks he knows better than you how much mana you are going to need on the next pull ignore him and drink to full. If he pulls anyways you will probably have enough time after you finished drinking to cast GS on him.
On low gear levels bandages can make a difference - sadly most dps players don't seem to bother leveling First Aid.
Most criticism you will receive will be uninformed and plain stupid - often people will use their half knowledge of your class to propose idiotic solutions instead of accurately describing the problem (and thus enabling you to find a good solution).
However, try not to be too jaded and angry to notice the valuable criticism - players have alts, off-specs, ... and might really know your class better than you do. I had several extremely embarrassing /facepalm moments during leveling and dungeons when I noticed that I had done some things wrong for a long time...
Since I'm not in front of the computer I play on, I can't check to see what my action bar looks like to see what spells I have to use. Off the top of my head, though, I know that I respecced out of having Spirit of Redemption.
I'm rarely jaded or angry at criticism, but how do I know what's valid and what's coming from someone who only thinks they know how to play a priest? Most of my problems lie with the tank running off after a fight to start the next one. I don't stand in the middle of a fight, with the exception, so far, of the fight with Slabhide because of the falling stalactites. So I have to run up to the group of dead mobs, loot the stuff that is mine and then run on. It just seems like the majority of the time people want to run a bloody marathon in dungeons.
I don't necessarily top off dps between fights, but I have noticed that if I don't and there's someone else in the group (like a druid or paladin) who can also do a little healing, they will top off other players.
Quality of life issues like that need to be pointed out to your party members, depending on the maturity you should probably adjust the tone a bit. What I've noticed is that in the 83-85 bracket, most of the douche bags hang out that get otherwise booted from regular heroics and zandalar runs. Try and cope till you can get beyond them or simply skip doing dungeons till you level cap. I remember in particular a couple runs on my prot pally where a couple DPS were being deliberate dicks. Leave the group if it's too annoying.
For the most part tanks set the pace and ones that aren't douches will slow the pace if you let them know you'll need a quick drink every couple pulls. Good tanks will track your mana bar without further hassle, bad ones you'll need to heckle with reminders.
As for healing advise, without seeing a healing breakdown it's very hard to give targeted advise. Generally speaking if you're experiencing mana issues, lay off Fheal useage and swap to Inner Will. Chakra Serentiy should be your go to state, and aim to keep Renew rolling on your MT with direct heals.