Hey guys this is my lux build this is my second attempt at it cuz it did look dull and boring and was very long. thanks for the comment man :). (sorry bad spelling at points it isn't my strong point)

Lux is an ap caster and kind of support with her Prismatic Barrier which can take alot of damage for your team mates and make other ulti's useless or less effictive like Karthus Requiem or Caitlyn Ace in the Hole you just need to use it at the right time's.

Personaly am not a fan of making full pages of the same runes like full ap for mp i think more could be done with other types of runes so you can have a more varied gameplay. I use this http://www.mobafire.com/league-of-legends/build/rune-tips-what-secondary-and-primary-means-and-more-24255 to help me decide my runes.
I got Greater Mark of Magic Penetration so my spells get past some of their mr at the start and it goes well
Greater Seal of Replenishment i do have some mana problems when playing with lux so this helps me along and once again it adds more benefit with Philosopher's Stone. HOWEVER if you like to play with Clarity u can change this to what u think is best either Greater Seal of Ability Power for extra kick or Greater Seal of Evasion if you want some defence early on.
Greater Glyph of Scaling Ability Power i this than Greater Glyph of Ability Power obvoiusly for later game and also it gives you a chance to catch the person ur laning agasint off guard if they arent paying attetion to those extra ap points as you level up and they find out the hard way.
Greater Quintessence of Ability Power once again just for power for early on and gives you 15ap at the very start.

Now the reason i haven't got a Dorran's ring is simply because i really dont like how it builds into nothing i think it is a lost item. They may be good at giving you a good edge at the start but most games you do get stuck with it till you have to get rid of it which cost's you about 200 gold maybe (just off the top of my head). So i get Boots for mobilty and gives me a bit of edge in lane as i can move out of range and still get those last hits.
My items mainly give me ap and not health thats because i always have the least amount of damage in the team i will explain why later. Lich Bane is one of those items u can get if you are confident with Lux otherwise i would keep it simple and get either Void Staff or Archangel's Staff. As i said before if you get Clarity you dont need to get Shurelya's Battlesong or Philosopher's Stone and get one of the items i just said.
If you do need health get Rod of Ages. DO NOT GET Rylai's Crystal Scepter IT IS USELESS ON Lux.

Most Important Spell

Flash well its Flash the number 1 escape spell and also the number 1 postion spell need i say more

Primary Spells

Teleport i get this becuase i can either help my other lanes quickly and get straight into action and also if i become low on health and i dont want to give them a free kill i can quickly get my items and get straight back into the lane.
Clarity This does solve Lux mana problem with one spell and also you go a bit mad in the lane with her spells so depending if i knew my team could hold i would get this.
Clairvoyance This can always change a game since it can always find that lurking player in the woods or an incoming gank or a lonely wee girl the river tryin to be clever and run away.
Secondary Spells
Exhuast now i wouldn't want this skill but it does slow the person and also it can take off some of their mr if you get Cripple for a mastery point.
Ghost Only take if you don't agree with Flash

The rest i wouldnt consider and you might have a go at Ignite but if you need ignite to finish your kill as Lux then you should have hit your Finales Funkeln LASSSSEEERRR

Now to play as Lux you need to work with ur team and also u need a bit of farm as your items are expensive. You deal ALOT of damage when ur items start coming in and playing there part. since your Laser cant hit all 5 of them if you can line them all up and also your Lucent Singularity can hit mutilple targets and slow them. Also you'r Light Binding can stop a tank from gettin to your carry or if you postion ur self right you can hit THEIR carry without putting ur self in damage and makin there team mince meat. Lux should never lead the charge for the team she is extremly squishy and more so with my build. What i do is i pick at there team with my spells and when the team fight breaks out i start out with Light Binding to stun the tank or carry and get one just behind them to make it effiect. Once engaged i throw Prismatic Barrier to my team mates in front of me who are takin damage then i hit as many as i can with Finales Funkeln followed with Lucent Singularity but not popping it untill i autoattack an emeny i know wont get or none of them, i pop and auto hit again. Once i do my starting combo i deal as much damage as i can with what ever spell comes up first and always throwing my Prismatic Barrier to my team mates that need it as it sheilds twice.
You are always at the back of ur team but no too far so you are away from the action but close enough so you can hit ur spells without dangering ur self.
Also if you see a team mate being chased and you are closet but know you cant get the kill for whatever reason dont just leave them cuz you can save that team mate as long as all 5 arent chasing but u know like 2 r cuz your Light Binding can bind two of them and your Lucent Singularity slows them so you can stop them gettin 350 gold and makes your team mate very happy to be alive.

Cheers for reading my guide any comments bad or good are welcome as i want to know what is either missing or if you wish to say what u think is wrong or if you know any way to improve my guide i will take into account but hope this helps you with ur lux game