Hi All!
I´m going to show you my way of playing Lux, the blonde laser mage. I play her mostly as a dps, but her spells are also good cc and supportive. Enjoy!

Everyone who has played Lux should know that Cooldown Reduction (I´ll now refer to it as CD), is essential. The ability to spam laser beams on fleeing or tower hugging enemies is so fun that yuo barely can´t live without it. Therefore, i choose to put flat CD glyphs in my build. That will give you 40% CD in level 18, where the CD is capped.Quints are flat AP to be able to deal serious harassing damage in start. For seals, i go with mana per level to be able to stay in lane longer. And as for the most of mages, i go with magic pene marks.

Not very surprising, i suppose. Going with 9-0-21 for the magic pene and getting the cooldown thingys in the utility tree. I go for the utility mastery points, because of the blue buff´s importance.

The items shown above is my usual build, but you´ll need to adapt this to almost every game. If you´re facing a hardcore ad team, for example you might wanna switch the Rylais for a Frozen Heart.
But the main build is starting with a Doran´s. You have to love this item for giving you a little bit of everything. Then the shoes are pretty obvious, i suppose, with CD being essential for Lux. That´s also why i choose to start building a Morello´s, since it gives you CD aswell as some mana reg and a nice portion of AP. Then there´s the Lich Bane. With Lux´s passive this is a powerful item you don´t want to miss. The basic stats giving you some ap, mana reg and mag res. But the passive of the item combined with Lux´s gives you basic attacks dealing hundreds of damage if timed correctly. Banshee´s is a nice defensive item with the passive shielding you, and the Mag res which is good facing solo mages, which often is the case in mid lane. You could get this item earlier if you´re having a tough time surviving. Then there´s the Rabadon of course. Not much to say. Ap, ap and more ap.
The final item is very adaptable. I usually go with Rylai´s for some health and AP, but also for the slow. A very usual happening is that someone´s standing under a tower with low health. You want to kill him/her by shooting in a light ball and then PEW PEW laser. The slow makes it much easier to hit with the laser. But other good ones for the grand finale is Frozen Heart, Archangel´s staff or if you feel like starting off with Mejai´s that´s just fine. Rod of Ages, Zhonya´s and Abyssal Sceptre are also nice choices.

The light ball(e) is one of the funniest and best harrassing moves in the game. You just shoot it, watch your enemy panic and run straight into it. Then you blow it off and hit him with a basic attack. The power is tremendous. That´s why i spec it full as fast as possible. Then there´s the q. It´s a really powerful ccing tool while still dealing good damage combined with basic attacks. It´s perfect when you´re being chased by two enemies. Just shoot it backwards and you´re free. I level it up fully after the ball´s leveled. However, I spec one point at the w shield. You could choose to put a point there earlier, especially if you´re facing a karthus or someting nasty.

I go with Flash and Clarity, since my item build doesn´t give you much mana early game. You can however swap it for Clairvoyance, which gives you the ability to spot a fleeing enemy and then PEW PEW. Flash is Flash. You want it.

Lux is a great midlaner aswell as an awesome support/ dps in a sidelane. However, you often want the mid. You shouldn´t really harass that much until level 3, when your balls start dealing serious damage. Focus on getting minion kills, as this is one of the hardest thing to do with Lux without spending mana on minions. By 3 and onwards, harass your enemy with everything you´ve got. However, try to get in a basic attack between the skills, for maximum damage from the passive. By six you should be able to ball+laser to get an easy kill.

Mid-game is the phase where you start to look out for your teammates aswell as yourself. If you notice there´s a fight on one of the lanes, you might just need to go halfway to one of the lanes and blow off a laser. When you start getting company in mid, both friendly and unfriendly, you should aim for harassing the squishees and disabling their melees. Soon you should be able to laser someone to death. Don´t go by yourself as this is the phase where their dpses start dealing serious damage, and you are a squishee.

End game is the phase where your spells don´t really deal that much damage as their team now should have good magic resist. If they haven´t got any, just keep on harassing. But usually your role in endgame consists of disabling their melee and carry characters, while shielding your allies. Focus your damaging spells on the weak ones.

Pros are following
-Extremely good cc with snare and slow.
-Spamshielding
-Ability to Laser fleeing enemies.
-Nice damage output early and mid game, especially combined with passive.
-Great harassing
-Short ulti cooldown
-Extremely funny
Cons
-Squishy
-Pretty useless in 1v1
-Hard minion kills
-Difficult to master
-Damage decreases late game.

You now have a light mage who´s tearing their team apart with fat laser beams. A good thing about enemies is the human factor. For your ball and laser, the enemies often panic when you aim and just run straight into your attacks. So, if you don´t have superhuman enemies, you´ll probably hit with pretty much everything. Feel free to comment and vote!

Thanks to KastanjKastarn and PizzaKnådarn for supporting me!
/ H4ll4B4ll4