Question about curses and dots

Hello everyone

I'm a beginning warlock and I'm aiming to maximize my dps and get to know rotations.
I'm playing as a demon warlock, which i'm really liking.

I was wondering how I should open my fights in dungeons/raids.
Right now, I start the fight by (pre-)casting Shadow Bolt and then Corruption right after while the bolt is flying. Then I apply Curse of the Elements and continue with Shadow Bolt/Conflagrate with proc... etc. (rotation is clear after the initial start).

My question is: Do I have to start the fight with Curse of the Elements instead (to get magic damage debuff on target first)? Or does it 'update' the damage on my already applied Corruption as well? I don't know how to test the difference properly, that's why I come here.
Opening with an instant-cast feels like a waste though, but if the debuff doesn't work for already applied dots, then it might be better to do that?

So in short:

SB - Corruption - COE
COE - SB - Corruption
SB - COE - Corruption

How should I open my fights? :)

I'm looking forward to your advice!
Thx :)

Since CoE is a raid-wide buff, you should cast that first.
It's kind of min-max thing, but when you think about it, on most bosses all the DPS's will pop their CD's soon as the tank pulls the boss, so when you apply CoE right away you are buffing them for extra 1-2sec (compared to SB - Corruption - COE) during the duration of their CD's (and overall fight).
These kind of min-max things are mostly looked at in progression, but even if it's not a progression you will remember it once you wipe on the boss while he has 1% HP. :)

considering you are Affliciton, you want to cast your corruption after target has both CoE and SB(or scorch, 5% crit) applied.
any raid-wide buff > personal buff ('tis why you might need to re-apply sbolt below 25% boss hp)
dont forget to apply CoE to adds too, if there is no UNH dk with ye.