So you wanna play Lux. That's cool! She's considered pretty underpowered, pretty trash. Even I consider her a bit of a watered down Morgana.
If you approach her the wrong way.
Approaching her the right way, Lux is what I like to call Offensive Support. This is how I build her, and this is how I play her. I've been playing her ever since I bought her some few weeks ago, and I've got down a method I really have been having fun with.
This is all opinion, feel free to criticize me.
Let's start before selection:
I would suggest Magic Penetration reds, Mana Regen/5 per level yellows, and Cooldown per level blues, with flat health quints.
This will give you the mana regen boost you'll want in early game, as well as the health to survive your older brother charging you through the bushes and trying to destroy your face, and of course the magic penetration to make him regret it.
As for Summoner Spells, I go Ghost and Ignite. Ghost is very good for just getting the hell out of dodge if the fan becomes clogged with fecal matter, rushing into position to set off that Finales Funkeln, or chasing down a straggler and Ignite is for those times that people survive your combo or try to tower-dive you thinking you'll go down like a Veigar or LeBlanc.
Flash is OK, but I find on Lux that Ghost has much higher utility
Exhaust is good if they have a good physical carry like Tryndamere. Screw that guy, he hates Lux.
Clarity.. If you're having mana problems, then just work on your play, because **** man.
Heal is good for that clutch save, or if you want to be a little more passive in your support/you lack a good heal or healer.
Fortify is just always a solid choice, I feel.
Teleport is good for setting up those lasers from hell.
Rally is wasted on Lux unless you really want to make your physical carry more stupid-awesome.
Don't get Smite.
That said, let's look at the champion herself and her abilities.
Light Binding - Lux releases a sphere of light that binds and deals damage to up to two enemy units, dealing 50% effect to the 2nd unit struck.
Fires a ball of light towards a target location, binding and damaging up to 2 enemy units. The first target takes 60/110/160/210/260 (+0.7) magic damage and is bound for 2 seconds. The second target receives 50% of the effect.
We're going to mostly just grab a point in this ability and ignore it for the most part. While it's got better damage than Lucent Singularity, I usually just use this to root people for 'the combo' or for getting away/setting up an ally's kill. Always remember you have, it, always remember it keeps going when it hits something, and always remember that it will save. Your. Ass.
Prismatic Barrier - Lux throws her wand and bends the light around any friendly target it touches, protecting them from enemy damage.
Lux throws her wand to the target location and back, protecting herself and all allied champions it touches from 50/75/100/125/150 (+0.3) damage for seconds.
Our Lux loves this ability and levels it, though she loves all of her abilities equally(except for one, we'll get to that later). It can/probably will save your ass from stray tower shots or give yourself/your allies that extra defensive edge to get away/get that kill/finish that tower before someone dies. Just remember that it's fairly weak. Very weak. Don't rely on it, it's mostly just a perk spell and something to do when you're waiting on your cooldowns.
But Garr, why don't we focus on Light Binding over Prismatic Barrier if it's so weak? Simple, levelling Light Binding just increases its' damage and (I'm too lazy/tired to check right now) its' cooldown. Levelling it last lets us use the points on this thing so our team is more resiliant. We're offensive support, but we're still support!
Lucent Singularity - Fires an anomaly of twisted light to an area, which slows nearby enemies. Lux can detonate it to damage enemies in the area of effect.
Creates a zone that slows enemy units by 14/18.5/22/26/30% (zone lasts 5 seconds). Can be detonated to damage enemies in the area for 60/105/150/195/240 (+0.6) magic damage.
What the tooltip doesn't tell you is that this gives sight. Lucent Singularity will probably be our Lux's most-used spell simply for its' utility; it's a slow, it scouts, its' recent range buff makes it a wonderful farming tool, it slices, it dices, it makes coffee. Oh and it's a decent area nuke. You'll be prioritizing getting this up, it makes a world of difference in a teamfight if you can nuke some people with this. It's also part of 'the combo'.
Finales Funkeln - After gathering energy, Lux fires a beam of light that deals damage to all targets in the area.
After a short delay, Lux fires a stream of light in front of her, dealing 300/400/500 (+0.85) to all enemy units in the area.
Our bread and butter skill. That's right, you heard me. Once you hit level 6, this thing should always be on cooldown unless you have no good place to throw it. It's an Ezrael ulti on crack with shorter range and a really give-away charge time, but the joy is in its' cooldown. Once you get your core items built, the enemy will be.. Well, let's just say "suddenly lux, wtf" will be something you hear a lot.
Illumination - Lux's damaging spells charge the target with energy for 6 seconds. Lux's next attack ignites the energy, dealing 20-190 (depending on Lux's level) magic damage to the target.
Lux's passive! This thing is a great passive, it synergizes with all of our abilities except Prismatic Barrier, and Finales Funkeln will activate it as if we had autoattacked, as well as re-applying it! Two-for-one sale at the Lux Emporium of Pain.
So those are our skills and basically how we'll be using them. Not bad, eh?
How about some item suggestions!
You should be a big enough boy to figure out what order you want to build these in, of course, so I won't include a step-by-step hand-holding guide to these.
There's your core. Now have some good extras:
Rod of Ages
Rylai's Crystal Sceptre(I'd recommend RoA over this, myself. I've tried them both, and Rod is just generally better.)
Mejai's Soulstealer(If you're doing well, this will beast. As always.)
Force of Nature(A faster, sturdier Lux is a Lux that hurls more lasers.)
So now that you know how I skill her, how I rune her, how I build her, how about how I play her?
Short and sweet, here we go. I'll elaborate if it's called for:
Early Game - Early game you're gonna want to play it safe. I grab a Meki Pendant and two healy pots(standard, really), and hurl singularities into the bushes or past the enemy melee creeps as the situation dictates, being sure to always blow it up before the enemy gets out of it. You want that damage, and if you can help it, always autoattack after landing one. In the early game you want to get a point in all of your abilities. As a supporter, you want those options and you want them -now-. Get wards so you don't get ganked, etc.
Mid Game - Lux stagnates around level 8, when mid game kicks in. Maybe you're done laning by now, maybe you're not. Regardless, you're at your worst here. Try to stick to at least one teammate at all times, use your singularities to slow or farm, or to weaken enemies in teamfights. Just stay back, the other champions are going to chew you up and spit you out until your core items are built and the best you'll do(unless you've been fed), is to bind them/slow them, shield and run away unless you have backup or they're low. Be ballsy, but don't get your ass killed.
Late Game - You've got your Archangel's and maybe even your Shroud, you're back in the game, baby! Around now you should be able to hurl lasers every 30 seconds, 25 if you're far enough along in levelling Finales Funkeln, 20 if you've got Blue Buff(don't hog it, you jerk, save it for the mages. Grab it if they don't want it, but leave it for pretty much everyone else but you. You've got enough CDR and by now you should not have mana problems). Welcome to the late game. Blow your ulti at pretty much every opportunity. Drop it on people from across the lane, use it to push(whole waves of minions will just disappear to a well-built Lux's Finales Funkeln) or use it in The Combo. The Combo entails the following in rapid succession: Light Binding, Lucent Singularity, Autoattack, blow the singularity, Finales Funkeln, autoattack if they survive, run away if they still survive that and you don't have your Ignite up or they're at half health, because you're out of ammo and without backup you could be destroyed in those few seconds you lack your spells.
-Try to stay in the back of teamfights at the tip of your autoattack range. This will put you in prime territory to bind or singularity people, while being back far enough to make shielding the people important a snap.
-Be unpredictable with your lasers. People expect Lux to use Finales Funkeln like Ezrael's ulti. Mix it up. Ulti a team right in their face in the middle of a fight. Zap them from an odd-angle while they fight an ally in another lane. Use it to steal dragon/baron/a buff if you have them warded! Use it to destroy the creep in a pushed lane to make them back away from the tower in a hot second. It is your best friend.
-Be judicious with your binds, and know the hitbox isn't stupid-huge like Morgana's. Try not to snare minions.
-Jungle if you can. With Prismatic Barrier and your low-cooldown spells, you can grab some easy farm.
-Always tail a teammate or stay on your side of the river. You're squishy and as offensive as you are, still support. Act it.
-Don't be afraid to charge in if people are weak. With your Glacial Shroud and shield, you should be able to snag a couple of kills if the enemy is weak and running away. Ideally you'll just laser them and they'll die.
Questions or comments or criticism? Post 'em! I want to hear and discuss. Have fun!