I think the hardest part for us Holy Priests, who have strictly raided as Holy, is learning how to heal as Disc.
You need to get in the mindset that you are trying to prevent damage before it happens. Thus, shielding 2-4 people in your raid, provided your mana can handle it, is the best way to deal with incoming damage.
Another way is to cast PoH on your groups before damage comes so that DA is up.
I guess... I'm not the best Disc Priest by any means. ._.
There are two Disc specs, however. Archangel/Atonement may lend itself to stronger AoE heals than SoS/ToT. My preference is Strength of Soul/Train of Thought as I feel atonement is very gimmicky but if it works for you then rock it.
Idk if you raid 25 man or 10 man, but here are my own parses. As a Holy Priest being totally lost about how to play Disc lolAshleycakez (alt)
- 25 man Normal MaloriakAshleycakez (main)
- 10 man Normal ODS
^ There are other kills that night (and other nights) where I played as Disc if you want to browse those as well. I just linked a normal fight.
As you can see, Disc is quite powerful and if you know how to play it well you can cheat your ilvl and perform very strongly.
For SoS/ToT spec (don't copy my alt priest's spec, its bad and I hardly play her - points need to be moved around), I just make sure I do my best to cast PoM on CD (if there is pulsing damage) and to Penance on CD if it is needed. Shield like crazy, and use Power Infusion on cooldown.