All u need to know is that your ult Finales Funkeln is one of the best ults in league of legends.
In the start u need to play defensive and farm alot minions with your e Lucent Singularity, then when u get to lvl 6 u try to do zoning and do q and e on the other champ/s and when they run and u see them with low hp do ult now then can do 2 things:
1. Do flash and then there flash is in cooldown.
2. They just die and u get the money.

This runes gives u some dmg armor and magic resist in high lvl but the main thing in this runes is the cooldown, in lvl 18 your ulti back in 24-33 sec (depends of items).
If u are in mid lane your seals and glyphs can be very helpful.

Farming is really easy with Lux u have your e Lucent Singularity u do it on the mnions and when they have low hp u bomb it.
and when u really need the money u can also do q Light Bindingand that thats good to.

This build is all about cooldown, hp and ap when u finish the builed u will be op and your ult will do 1000 dmg and is cooldown will be really low.
now the items are really good it gives a lot of ap and alot of hp and mana but cooldown is the main thing in this build.

Well flash Flash is really good to chase and for the start of the game when u are lvl 6 zoning u will do flash q Light Binding e and then ult.
Clarity Clarity is one of the best things for her because u are doing alot of spells in no time.
U always need clarity Clarity but u can change flash Flash to ghost Ghost if u are bot lane ghost Ghost can be helpful but I think that flash Flash is better.

Team work is really good always but with Lux it can be very helpful to have a good team that listen to u, u can tell them to do some dmg to an enemy champion then u do ult and get the kill and they have assist.

Lux is one of the best champions in league of legends she is a really fun and funny champ when u buy her u never say why did I bought her and u will have so match fun we her.
her ult Finales Funkeln is really OP and it goes very well with her q Light Binding
and her e Lucent Singularity her w prosmatic barrier is to run from the enemy and good vs cait and nidalee.