Shadow priest revolves around managing DOTs on the target and has a rotation that close resembles that of an Affliction Warlock.

Rotation for Shadow Priest Wotlk

Shadow Priest Priorities

Vampiric Touch (Keep debuff on boss) > Devouring Plague (Keep debuff on boss) > Shadow Word: Death (At 35% Health and on opening) > Shadow Word: Pain (Keep Debuff on boss and apply with 5 [Shadow Weaving]) > Mind Flay (Filler)

Tips

At end game Mind Blast is used only to increase [Shadow Weaving] if needed, and it’s dropped out of the rotation

At the end of a cast an instant spell can be cast without occurring GCD, this is true for every cast/instant sequence, should be used to maximize DPS.

Use CDs when there is barely any movement or crowd control effects to maximize DPS, best way to do this is by knowing the fights.

The addon NeedToKnow can help track all the debuffs.