Please read these two recent threads which explain a lot.
https://www.mmo-champion.com/threads...r-Druid-issues
https://www.mmo-champion.com/threads...4#post48213264
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And your specific questions
when to use my CDs properly?
Its hard to say exactly when but best tip I can give is to know the combat and expect incoming damage to use them.For example, Torment cats of Argus and its CD fits perfect with WG. You can keep rejuvs rolling and once Argus is casting Torment, you start casting WG with 1 sec delay and voila once the damage hits you have rejuvs WG's ticking and you can either extend it with Flourish or use G'hanir for higher output or both. Or you have rejuvs rolling you WG and you need to move (i.e. Kin'Garoth) you simply Flourish to extend hots on people while you re-position. Or you use G'hanir if your Tranq is on CD and there is heavy damage. So they are situational and if you know the combat you will know when to use them.
When to use correct spells
Swiftmend is used for emergency heal. Not much to write about it unless you use T20(you don't) or the Soul of the Archdruid ring(you don't) which interact with Swiftmend.
WG must be used AFTER damage because ticks of WG hit hard at start and heal less near to its finish. For example you can time it perfectly with Argus Torment cast. You start casting WG after Argus starts casting Torment and you land your WG right after damage hits. You can cast a few rejuvs and extend the hot by Flourish almost before WG finishes. This usually carries you until next torment. You must know when there wil be damage. If not simply you react during combat.
Which trinkets
Darkmoon deck promises/Astral Alchemist Stone/Carafe of whatever(Varimathras iirc) for mana regen. If you play with high output you will have mana issues. One of these trinkets can help you manage it better.
As for second trinket if you have Velen's and can use it to full its
THE BiS. If not any trinket with high intellect (3k+)+ mastery/crit/ver (they are called stat sticks) is good enough. It will not effect your output much even if you have an inferior healing trinket. I use Carafe with Deceiver's Grand Design and in some combats just swap it with my mastery+int stat stick which is mediocre but whatever.
Which talents for M+ and raiding
Cenarion Ward (more to explain on this) Cultivation Germination and Flourish for raiding. No need to explain these.
For M+ I recommend Incarnation, germination and Moment of Clarity and Abundance on first tier. Need to explain these choices here a bit. Please keep in mind this is all for above +15 even above 17/18. Anything less you can do with whatever set up.
Since you are the only healer in M+ there will be those oh shit times. This is where Incarnation is more handy then Cultivation. Moment of Clarity (3 clearcast procs) goes very well with Abundance. Abundance increases your regrowth crit chance for each reju you have rolling around. With germination you will be running at least 3-4 rejuvs any given time possibly much more. This means almost a %80 crit chance for your clearcast regrowths. MoC also increases healing of these regrowths by %15. So you are criting around 2mil free regrowths and you get some crazy numbers when you are in Incranation and buffed by Alchemist stone etc. So all these talents click like gears and work in harmony for M+
I don't get why people use Cenarion + Cultivation
In an ideal setup with 2 or 3 healers healing, cultivation shines. Because it procs only on people with <%60 HP, its a very high buff to you with less healers and where raid dips frequently to %60 HP. But if you are running 4 healers Cultivation will be simply redundant.
Cenarion Ward. Its a fire and forget spell with a shield which you either use on yourself for survival or on tanks to make max use of it. If you can extend it with Flourish its a much better tier 1 talent then others in terms of output in a raid situation. But usually it ends up being forgetten or redundant as healers snipe each other and it simply overheals.
So you can choose any tier 1 talent, but in theory CW gives more output.
For cultivation if you are running with min. number of healers (i.e. 2 for a 9 dps raid instead of the regular 3 for any given pug) it will outperform other talents in that tier. If not whatever works for you.
Here is an old thread regarding M+ and resto druid.
https://www.mmo-champion.com/threads...0#post46471740
Also check out MadSkillzzTV
https://www.youtube.com/user/JuicyFruitWoW1 for lots of resto druid guide videos. His videos helped me a lot to improve my healing.
Oh btw if you are not using, use a raid frames tool such as Vuh'do( I use) or Grid in combination with mouseover heal macros. This way you simply mouse over to injured unit, right click for rejuv, alt right click for Lifebloom etc. (these are adjustable in Vuh'do or Grid) If you are not using this let me know and I can give more information on this. This play style will help you immensly rather than clicking each target and casting each spell.