Healing is about know the encounters and its timers, to know when to use what, predict where the damage will land and react to it.
Healing a encounter is to know what your class can and cannot do, what is in your toolkit and when to use it effectively and save mana (sometimes its all about to use a CD because its not needed for the duration of the cooldown because the HpS/mana is better than just heal for instance).
So.. in the light of your question, know your class and know the encounters, that will give you the first set of tools to work the fights for at least 2-3 tries till you get the hang of it and evolve from the initial knowledge to the "field knowledge"
hope it helped, because... well... healing on a boss basis, that I know, does not exist since there's sooo much randomness on certain events...
I'd say just check a ranged dps guide for positioning and read the damage that abilities do in the dungeon journal. I found that that's all I needed to know to prepare for a fight since you'll have plenty of time to learn the fight while the dps is figuring out that that they shouldn't be standing in fire.