I accidentally hit publish and cant take it back.
Grammatical mistakes ect to follow, but you should be able to glean the foundation of the build.

This is a long read and gets more into game theory than actual stats ect.

Hey there, this is my first guide blah blah blah, so rate how you want to and if you dont like the guide or have some constructive criticism, Im man enough to take it, so leave it at the bottom. I generally play normal games with at least one or two friends who have as much teamwork as a flea. On the rare occasion that we do work together, its my job to keep some coherency in a team fight. GOOD MAP AWARENESS IS A MUST. I CANNOT STRESS THIS ENOUGH.

This guide will have no hardcore stats, but gives you a general overview on how I like to play lux and probably a good 25% of all of my 400+ games have been played with her. My build focuses on a team oriented damage and utility build and possibly carrying late/long games. The soulstealer is the linchpin of this build. Your team can be getting slaughtered, but if you JUST STAY ALIVE and keep getting assists, your going to watch with glee as the tide of the battle turns your way. Assists will get you that stack, so even if your buddy is chasing some overextended hero on the sides, feel free to throw in an FF just for the assist. Dont be a douche and steal it though unless its obvious that the enemy champ is going to get away. Lux already has too much of a reputation as a kill stealer. Try not to perpetuate this.

PRO:
Great practice for those who lack map awareness
Humbly pushing your team from the back (a very rewarding playstyle IMO)
Shines late game if the enemy had good mid game momentum.
Seeing the stats bar at the end is always a treat (damage taken should be almost zilch)
Ulti will take half a squishes life late game

CON:
If your team isn't doing its job, neither can you (this applies to all champs though)
Squishy as hell
Cant go toe to toe with anyone
Requires impeccable map awareness

MS runes were expensive as hell to buy, so I stuck em on all of my champ builds. I personally cant see any reason not to have these and I love them to death. If you wish to substitute for something else, such as CDR (I dont have a ton of that in my item builds) feel free.

Magic Pen marks: The great AP flat vs Magic pen debate. Well here it is in short: late game, magic pen is better until you cap it.

Glyphs: probably the biggest problem with my build is CDR, but frankly I dont find it too debilitating as long as you have these and get blue pots when you can. Some of you have already looked at my masteries and thought, WHY U NO MONSTAR BUFF? Here's why, its not your place as an AP support to get it unless you were pre nerf sona...alas those were the days.

Seals: These could go on any hero that uses mana and I wouldn't care. Great runes all around and they keep you in lane longer.

Standard stuff. I have to make a note that the clairvoyance mastery plus utility reduction on summoner spells is almost OP in my opinion. All the CDR in this build is absolutely brilliant. It really makes a difference.

I take the gold per second because frankly you wont get much in the way of farming, and champion kills aren't your style early game.

The soulstealer. It is everything that this build is based on. My first 20 so games as lux, I was afraid of getting this item, because the fear of dying was too great. This instilled in me the best mind frame one can have in this game if playing a squishy caster like myself: kiting to not-death. A recent list came out regarding the top solo ranked players and the common denomination was this: they could hard carry, and had very high mobility. Now its pretty rare to get lux (or any caster) to be a carry due to MR and cooldown restricting ability spam, but a slow, snare, flash, and good map awareness should be more than enough to keep you alive. With all of this in mind, my items are all based around hardcore AP/magic pen. The tear should be held off from conversion to archangel staff because the full AP effects wont be appreciated until you have proc'd the tear enough to build a substantial mana pool. You may notice that you will go over the magic pen cap with the void staff. If you desire, you can change the mark mastery to flat AP or you can exchange your boots for the lucidity boots for some much needed CDR.

Some people love to use the crystal scepter, but I find the slow to be a little lackluster. With your Q, the snare will outlast the slow in general, and the ulti has too long of a charge up to be used for post use chasing. Your W has a slow already, and your E wont

I've never seen a game that went to me getting a litch bane because I'm ALWAYS buying wards, but it must be mentioned that if a game were to get this far, you may as well get it.

Your snare doesnt give that much damage, so your better spent getting the snare effects and then pushing the shield up instead. The rest is self explanatory.

Flash keeps you alive. It is your get out of gank free. No amount of love can be showered on this skill. Learn to use it well.

Clairvoyance:

1. Check a brush for ganks (self and teammates)
2. Barron/Dragon checks
3. At the beginning you can check lane enemies by throwing it right on top of the nexus and watching where people go. Keep in mind they may switch after the leave vision to counter this.
4. If in a side lane and the enemy is in a brush AND the minions havent engaged some target yet, you can divert their movement to proc on the enemy champion. Early game, this can actually do some decent damage.
5. You can stop an enemy from doing a potentially embarrassing juke by throwing it over the brush while chasing them.

The spell will not stop you from blue pilling back, so if you are porting home dont feel bad if you think you need to use it for an allies lane, and dont just use it in your own lane. A quick assessment of the minimap can give you an indication to use it for an allies sake.

This is one of the harder thing for lux to do keep in mind two things:
if a teamfight is not in the near future, an ulti can decimate an entire creep push. The cooldown on that thing is short so dont be afraid to waste it every now and a again! Also once your E hits level (3/4 depending on AP) you can use the passive proc to one hit a caster minion in one shot. If still in lane and the harassing is pretty tame, this is the best way to farm with lux. Her Q and W should never be used to farm. It wont work.

For two reasons, Lux has to keep the rest of her team between her and the enemy:
for one, her abliites all work better if the team is accumulated on a certain point. Particularly her W. This spell will be used two times at least per team fight, so dont forget to use it. Also, if you use your ulti early on (though it may not score any awesome kiils) may allow you to use it again soon after the fight is over to score a kill on a fleeing enemy or if your team got wiped you can slow down the impending counter push by constantly harassing the enemies advance. Remember, even if your team ought right SUCKS, you can still salvage a win by just getting the odd assist and not dying.