If you're playing Support Lux you're doing it wrong.
Ability Points go Q -> E -> E -> W -> E -> R -> E -> Q -> E -> Max Q -> Max E
Items are either one or two Doran's into Rabadon's (or Mejai's if you're consistent enough, since Lux can abuse it), into Void Staff into Lich Bane. Also Sorc boots in there.
For your burst I generally find it easier to get everything off if you go Q -> E -> R -> Pop E to make sure they die instead of Q -> E -> Pop E -> R. Your ultimate is also an amazing stealing tool. I stole a baron once and just a few days ago I stole a dragon completely blind. Make sure to use your shield in teamfights because it does surprising amounts of good. Also make sure to remember you can snare people behind minion waves if you aim your snare right, but poking with E is generally the best idea (and really easy). Most of your CS with Lux will be autoattack last hits until you get enough AP to clear caster minions with E (which may not ever even happen), but at Rank 5 you can clear caster minions with an E and an autoattack on each to clear your passive procs. Make sure to abuse your passive procs if you can do so safely. It does a surprising amount of damage.
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~Acting on Impulse~
Black Turtle still didn't MAJORA'S MASK