Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This is basically the Lux build I like to use. Lux is a great champion to harass and make the enemy second guess their actions. She is very squishy so make sure to stay back and attack from a distance. Your main three items would be the Sorcerer's Shoes, Archangel's Staff and the Nashor's Tooth. After you can change up the other three depending on what the enemy is doing. You may need to throw in a void staff for some more magic pen.