Hello.

You'll probably be downvoted a lot because there is hardly any content or good advice that you give that can't be found in the League of Legends Wiki. Most of the guide is merely a summary of Lux's skillset and an item path suggestion. You should try to add some sections (like a Composition chapter, explaining when you should and shouldn't pick Lux), an in-game section, talking about her fortes and weaknesses, and so on.

I also think that some of your item choices are questionable. Lich Bane is very bad for Lux since it counters the kind of play you should try to aim for with your kit (very safe ranged poke - while Lich Bane needs you to get close and expose yourself to your opponent.

The Seals are also bad: you'd be much better with Greater Seal of Armor. Flat MR on Glyphs is worse than Greater Glyph of Scaling Ability Power as the breakdown in which scaling is better comes at level 6.

I won't downvote you since I think that you don't have any fatal flaws that can't be fixed quickly. I advise you to archive the guide before rewritting it so you won't get quickly downvoted.