There is no introduction I`m not a good talker I just craft best guides, do not comment about my english , so I won`t comment about your skills

The runes provides you sustain versus BASIC ATACK AND AP HITS, You only need the 15 AP Quints Because the build provides sustain and harass over lane nothing to talk about here this are basics

The Q is your main skill for harass, engaging, and running after targets.Things to know:
You can Q and INSTANT R because The R gets reset if Q LANDS ,practice Q R lands Q have -20 velocity of R so it`s almost human imposible to hit faster the R then it lands the Q even at smartcast, BE AWARE OF YOUR RANGES
You can use Q and R to follow targets out of range by just Q and R on a minion
This skill have a low mana cost , in combo with Philo and Spirit stone you can continue harass with no worry of your mana
You can use Q and R to juke Ult`s , A good player it`s not only harassing and killing
You can Flash Q R for long distance kills

The W It`s Your main protect skill. You use It for protect or for executions on 1 vs 1 as Q R LANDS You Use E and then W so all damage is applied by exploding all 3 balls at once , but you get less shielding
The jungle Diana, starts with the W and in order to jungle better start with a 435 book and 1 potion, spectre and smiteless red, head directly to blue and do it, buy kage , philo and same build after
Try after 2 balls of W hits, move back a little so your 3rd ball does 1 more rotation you can compress more of your shield

THE E ITS YOUR MAIN FARMING SKILL GET A WAVE OF MINIONS PRESS E W AND Q IN CENTER.You can do this from the lowest levels to deny the xp of enemy who leave lane so you force to stay on lane , or to simply push. AT a time of game where your ap is very high you just give E and Q
THE PASSIVE IS YOUR MAIN SKILL Before atack keep the 2 hits, you can hear specific sound for the second hit
THE ULTIMATE NOTHING TO EXPLAIN OF IT, Do not use it twice try to compress to the Q and ULT Even if you slain the enemy faster you can see your ult It`s down and then its hard to fight another enemey

Diana can easily baron Duo with the 20% vamp item , Lichbane and a rabbadon, As soon as you get Zhonya and nashor tooth you can solo
You can even solo with the lichbane rabaddon 20% vamp but it takes quite of skill and a blue buff

I wouldn`t Play:
diana with a Teemo on lane ( Everyone hates teemos)
Casiopeea it`s strong vs Diana
Malthazar Very strong
Kassadin Again Biggest counter ( cause of silence ) Why kassadin strong instead of what says lolcounter ( If you want atack him he silence your dead, if he atacks you he E W and if you Q he silence you dead
Talon ( AD SILENCE ) VERY STRONG vs Diana
Chogath HARD TO KILL DAMM
Fizz best diana counter