Diana is a strong burst assassin thats easy to play. She is Mana Hungry and Squishy early on. I start my build with a sheen or a catalyst stone depending on the team comp or matchup. crystal flask is a must have early on in order to trade and be able to keep health ad mana up for an all in. harrass every time you come back to lane with Q, this puts enemys in a position where they cant afford to all in you, and you in a position where you can ulti to them if you feel like putting boot to a$$.

Fights/Ganks-
Never all in before Level 6. always hit your Q before using R otherwise DO NOT go in. get ahead early and you will snowball. if you get 1 or more kills before 7 and you aren't killing your lane partner or ganking other lanes YOUR WRONG. If your ahead or your strong early do something with it. your an assassin, go assassinate $h!t. if your jungle has strong CC you can R straight to the target and use moonfall, its a good way to catch them off guard. Never be the first to engage unless you have an Amumu or a fiddle to follow you in and capitolize on Moonfall. also if you use your super moon shield and then ulti to something it will pop all the little orbs its very effective engage.

Escapes-
if someone is riding you to your turret use moonfall it does bring them closer BUT it slows them so you can use it *** an escaoe. also Q>R>Q to minions can make a huge gap between you and the noobs that think they can kill you.

Role-
You are a powerful kamakazee Assassin who can ruthlessly slay enemy carrys quickly with your strapon of doom. when the team begins to fight QUICKLY Q and rush the ADC or APC and bring the pain. dont forget that if you still have R up and you killed the carry your ulti can provide you an escape to minions and whatnot.

Items/Runes/Masteries/Spells
these are what i use because they work FOR ME. do whatever works for you this is just the most effective path i have found.

Your an assassin act like it. if your Ahead murder your lane or someone elses lane.

Building lots of AP makes you more effective. destroy ADC and APC in team fight.

your not a tank dont be the first person to go in UNLESS you have a fiddle/amumu/malphite etc. in which case you buy zhonyas golden mode instead of death condom first.

always get your mana item first. you need it more then anything. if you have full health and no mana then your a useless turd kill yourself.

P.S. something helpful when your jungle ganks auto attack a minion twice then Q>R>Shield>Moonfall etc etc you get the passive strike with your lichbane right off the bat makes a powerful burst.

You have now mastered diana. Go play ranked right now and 1v5 win all your games.