Masteries, Runes, Summoners
14/13/0, ArmPen Marks / Armor Seals / MR Glyphs / ArmPen Quints, Ignite/Ghost
QWQE (ranged) / WQQE (melee), R > Q > E > W
Flask, Cloth Armor
Blade of the Ruined King, Ravenous Hydra
Mercury Treads, Ninja Tabi
Situational Supplemental Items
Brutalizer, Phage, Hexdrinker
Late Game Items
Black Cleaver, Last Whisper
Frozen Mallet, Guardian Angel, Mercurial Scimitar, Randuin's Omen, Maw of Malmortius
Most important things to work on are how and when to trade in the laning phase and how to team fight. Q combos (Q > auto > wait > Q > auto, QQ > auto, Q to creep > Q to target > auto) are incredibly important to master as well. When are going to attack a champion, make sure you have a stack of the passive as well as every little bit can help. This way you reach the Duelist cap faster. Practice makes perfect, and experience is what gives you the knowledge of whether you can trade or not successfully. She a lot better in trades then most realize right now. Team fights are effectively about assassinating the AD Carry or some other squishy high-value target. There can be some choice in determining whether to use E before or after a Blade Waltz, but in general you should start a team fight by coming in after the team fight has started (preferably from the sides) with an E -> Q -> W -> auto -> Q -> auto until low on health -> Blade Waltz -> activate items on last slash -> hopefully you can reactivate E now -> kill everyone. Obviously, not always going to happen, but it's a place to start.
Adapt your build to the game as well. What I wrote isn't always going to be true. If you need faster percentage penetration, then you need those items earlier. If your opponent is going to fight you melee all the time, then maxing E is probably your best bet. (Do not max W first unless you really know why you are doing it, in most cases melee stand-and-fight trades do better with E and ranged trades are far better with Q.) Adapt to the game. Change your runes to something defensive if your opponent is going to wreck you. Adapt, practice, and learn.