Before the pull
First, don't forget to put your Onyxia Scale Cloak on. Don't forget neither to buff Shadow protection, as there are only a few boss that requires it and Nefarian is one of them.
For the P1, the raid is divided in 2 groups and you'll have to assign healers for each of them.
You'll also need 4 healers to go heal the tank directly at the landing (P2). There are still some mobs from the P1 at the landing, so don't all go to the boss!
Lastly, if you are a dwarf priest, use Fear Ward on ass much poeple as possible before the pull and during the P1.
This is a very intensive mana fight. Use consummables and have mana pots.
The Phase 1
You can't let anyone die during this phase: shield the squishy people taking aggro, use flash heal if necessary, ect ...
Don't bother to dispell the 4s duration debuff. But do dispell the fear and sheep on MC'd people.
If you are MC, pray for the mage to sheep you quickly, otherwise it will burn your mana.
If you are a dwarf priest, Fear Ward the more people you can.
The Phase 2
When Nefarian lands, the raid stacks on one side of Nefarian. You should be at max range to avoid the AOE fear. If Nefarian moves a bit, adjust your position to be max range at all time. You should never get feared if you're positioned right.
Each minute, Nefarian makes a Class call. There are 4 class calls that will have an importance on your gameplay :
It's very important that you don't throw a single direct heal on anyone during the priest call. You can still use Renews and Shields.
During Warrior Call, the MT will need extra heals. During Druid Call too, because the druids won't be able to help.
When the infernals of the Warlock Call get to the raid, use fade immediatly and try to stop healing for 1s. Otherwise you may take aggro and die. Also pre-shield mages, that will aoe them and may also die from them.
It's important that you have the timers of Class Call. During priest call, the MT often gets 2 dots because some priests had started casting before the call and that the heal went out just after it. It shouldn't happen, so watch this timer very carefully.
The other important timer is the AOE fear, especially if you're a dwarf priest. If you are dwarf, you'll have to rotate with the other dwwarves to Fear Ward the tank after each aoe fear.
The Phase 3
At 20% HP, Nefarian will summon tons of skeletons. Cast fade instantly so you don't get aggro. Shield the mages.
Besides this one summon, the P3 will go on exactly the same way as P2 : watch the class call, outrange the fear and heal the tanks.