The basics
Preservation is a somewhat complicated healer. It's heavily based around the usage of several short-cooldowns and the empowered spells, it lacks a real useful spamable healing button, has a shorter range than any other class and it weirdly goes from having a lot of mobility tools to also being constantly inmobile casting long channels.
Using just the right amount of resources to respond to any given situation without overcommiting cooldowns that you will need soon is a key aspect of succeding as a preservation evoker.
Empowered Spells
Preservation has three empowered spells: sb, db and fb
sb is a single hit high healing spell that goes from single target to up to four targets depending on how high you empower it. It also has a really bad tooltip. The healing doesn't actually split, it does the full amount to each target it hits, and higher empower levels add more targets to it. When you cast sb on a target, you send a single bolt to that target, this bolt then jumps to all the extra targets simultanously according to the empower rank of the spell and chooses them in a fully-smart way. This means the lowest HP players are prioritized.
Because of mastery, you always want to be at higher health than the people you are healing, sb plays into this in a really neat way: if you send sb into yourself, the main bolt will heal you before choosing the extra targets, this means you will most of the time have your mastery active on everyone else hit by it.
db is a frontal cone aoe with both an initial hit and a heal-over-time. The higher you empower it the more healing the initial hit does in exchange a shorter over-time heal. db gets a huge buff from ve via the coy talent, and so you always want to use ve before every db.
You will mostly be using sb when you are in need of instant burst healing, and db to cover rot damage.
The third empower, fb, is a damage button. It empowers in the exact same way as db. fb is still an important part of our healing kit because of lgf and leaping.
Casting fb will give you the leaping buff, which makes your next lf cast one extra flame per level you empowered the fb. These extra lfs can proc their own bursts and will be aimed at enemies if your lf target was an enemy or at allies if you used it to heal. If you cast lf on an enemy with leaping up but theres not enough hostile units close enough to target with the lfs any extras would heal nearby allies.
coy doesn't work with leaping, as only the hard casted lf would get its benefits. Other static effects like chronof and lfm do benefit all of the leaping lfs though so it's still a potent aoe heal you can use and part of why fb is still relevant when talking about healing.
The other part is lgf, which excels in aoe scenarios. On mythic+ unleasing a max rank fb on atleast five hostile targets it's enough to heal your whole party back up, making it just as good as our other two empowers at healing your group.
During raids while playing echo builds, most of your healing will come from spreading echoes on the raid and then consuming them with a powerful spell. The most common spells to do this with are sb and db. fb can't be echoed on the account of being a damage spell, but leaping is still very important as it generates a lot of extra bursts to let you cast even more echoes. You will mostly try to use them on cooldown, while aiming to have sb cast right as a big damage event hits the raid, and db to cover over-time healing.
On mythic+, all three of your empowers are your primary source of healing, and you need to use them smartly and space them out so you always have atleast one to respond to damage.
echo
This is the key component of the spec. When you cast echo on someone, the next healing spell you use on any target will be replicated on that person, consuming the echo. Covering your group with echo and then casting a spell is how the spec uses it's resource to increase it's healing and to cover more targets than what the spells are normally capable of. Good echo management is key to playing preservation well.
For a detailed list on all the ways echo interacts with the rest of our spells you can check the spell interactions page on echo.
ta
Another fundamental part of the preservation toolkit is ta combined with reso. This is the tool that lets you quickly cover people with echoes. Notably these echoes are weaker than ones applied by manually casting the echo spell: they replicate spells at 30% effectiveness baseline and 45% with timelord, as oposed to casted echoes which do 70% baseline and 105% with timelord.
The tradeoff here is that ta lets you apply echo in a very quick and efficient way, not using essence at all, but in turn do less healing. In raids you will combine the two types of echo to cover as many people as possible before you use healing spells, while in mythic+ you might choose between one or the other depending on a lot of factors like how much time you have before needing to heal, how much avaliable essence you have, or what spell you are casting to consume them.
All the interactions that work on echo do the same thing to echoes applied by reso, they simply do smaller numbers.
rev
rev is a spell that on its own is just a heal over time that gets extended on critical hits. The complexity of this button comes from all the extra effects it gets added to it via talents, like gh, lifespark and gp, on top of the resource generation it provides by having a chance to give us burst on aplication.
During raids rev isn't really a very usefull spell, and isn't valuable enough to consume echoes with. It can be used as a spot healing tool via gh but besides that it's not very important to press it a lot. On keys however, rev becomes a very important part of our kit. echoing a rev applys two ghs to the target, quickly letting you recover someone from a spike of damage. While blanketing the party with ta and reso gives you a nice group-wide healing buff on gp. On top of that lifespark gives you a constant stream of instant lfs that can be used to increase your dps or to quickly heal single targets.
For more information on how rev interacts with the rest of our kit you can read the spell interactions page.
ve
ve sometimes has the problem that it does too many things. As this is both our coy and lb applicator plus a mobility tool and also a big single target heal. Most of the time you will be casting ve on yourself, in part to avoid the danger of flying to another group member which position you might not be sure its safe and in part because you will already have echo on the everyone you want to heal and thus they will get healed by ve regardless of who you target.
As mentioned before coy is very important to have before using db, and lb is also a huge part of the spec in general, letting you transfer healing on yourself to all your connected allies if they have echo on them when you cast ve. The main thing you can transfer is ec, which is also one of our strongest healing cooldowns, as transfering this huge amount of self-healing to several people is incredibly potent.
For a detailed way on how lb works, you can check this archive page and the spell interactions page on lb.
blossom
blossom is a very unique spell that has very niche uses during normal echo-based gameplay. It's good to consume ouro and burst with it when the correct situation presents itself: A clumped group of players needing healing soon. But besides that it's not a button you should press very often and on mythic+ you can get away with not pressing it at all.
However, on raids there is a style of gameplay that revolves entirely around blossom and uses all your resources on it instead of echo. These "Blossom builds" use specific interactions between our talents to maximize the healing you can get out of blossom and abuse af to cast quick lfs to generate burst and be able to use more free blossom.
This builds have a really different healing profile, doing consistent flat healing over the duration of the fight instead of the peaks and valleys that echo does where you don't heal much while spreading echoes and then burst a lot when consuming them. Using blossom a lot is also more mana consuming and doesn't lend itself to long fights or fights where you need to concentrate your healing on specific points.
Cooldowns
Your main cooldowns are rewind, df, stasis and ec. In the case of rewind and df they are very basic "press and they heal" style cooldowns, df does require you to aim the group where you want to send it but that shouldn't be a problem.
ec has the extra requeriment that for it to be a raid cooldown you need to first apply lb to as many people as possible, again in raid this is done via combining reso and as many regular casted echoes as you can, while in mythic+ you can use either of them with manual echoes being more than twice as strong.
stasis is a more complex spell. It remembers your following three casts and then casts the abilities again in the same order you originally did and on the same targets when applicable. It doesn't save buffs you had when you first cast the spell, and it will consume buffs during the release stage, like coy. For a more detailed explanation of how stasis interacts with your spells you can check it here on the interactions page.