Welcome to my first build,
this build is about Lux, my most favorite champion in the whole game.
To use this build you need to harass well, if you cant do that please stop reading this guide.

I pick my summoner spells for both flexibility and staying on lane as much as possible, clarity gives you and your team mates mana, and ghost is good for chasing or running away.

My Runes are very different from normal caster runes,

Flat MR Marks; this is for survival, if you will have to go back to base every 5 minute because you get harassed by the other team you wont get far with this build, you want to go back to base at level 4/5 not 2.
Flat Armor Seals; Read marks.
Flat AP Glyphs; these give most ap out of all runes, therefore i chose those for ap, they increase your damage at the beginning.
Flat health Quints: those quints are good for all champs, Lux ain't any different

My masteries are pretty standard for a caster going 9/0/21 in Utility.

I start of Doran's Ring + Health Potion, this item is great for beginning and i leave it till i will be getting the last item, depending on how good the other team is, i get either Mejai's (if you are getting kills) or Rabadon's Deathcap if they play more safe, you get 1 item after the other anyway. then I get Zhonya's Hourglass as the passive is great + the ap. The next item is Rylai's Crystal Scepter, as it both hp and ap with a good slow aswell, followed by EITHER Abyssal Scepter, if you have lots of caster on the other team, and even the squishy are buying MR, or Void Staff if their team is based on 'tough' champions (poppy/nunu/garen)

Early game I suggest you simply kill minions, at level 2 you want to throw your E whenever possible, Q when you will know that you will hit. when hitting level 3 use your W ability when your other team mate is attacking enemy champions, REMEMBER ! dont start team fight on your lane, you will die.
Mid game is pretty much the same, you have your ulti now, but remember its not for harassing, its for killing, if someone stupid stays on the lane,use your ulti to get a free kill.
End game, now that you have pretty much all your items, and the team fights had begun you want to stay back off a bit for your tank to take the damage, and join in 2 seconds after the team fight has begun, otherwise the enemies will focus on you and you will probably die, REMEMBER use your ulti to kill, not to harras or in the middle of the team fight, if your fed you might as well score double/triple(quadra?) kill!

Currently in edditing

Thats it guys, i might make a video or I might not depending if i have time or not,
pls comment and rate.

Lastly, sorry for my English although I tried my best, I'm pretty sure I've made some mistakes.