Alright so this is my first Diana guide, Since S3 just came out I feel as if many players wonder which build to follow / masteries.

For runes you can either take Magic Pen Reds, or Ability Power Reds.
I usually get AP Quints along with MPen Reds in order to maximize damage
Some people like to run AP Per level blues, but I feel as the M Resist is really useful especially when playing mid.
Armor yellows

PROS
Scales Extremely well
Massive Burst
AOE Slow
Easy initiations

CONS
Requires good farm to do well
no sustain
Small boobs

Sorcerer Shoes : This gives you the Magic penetration needed since we are not getting a Void Staff. You may switch these out for Mercury Threads for the tenacity as CC can really mess you up and render you useless!

Rabadon's Deathcap: A no brainer for Diana. This Item gives the most additional ability power out of all items in the game. Your going to want to rush this early. Getting all this extra AP early on is a much better choice than going for MP first for a number of reasons. Most enemies dont start getting substantial MR until a bit later on and also like i said Diana snowballs like crazy, thats why you want the strongest early game possible.

Abyssal Scepter: Since you're always going to be close to your enemy this gives you nice Magic Resist, AP and Magic pen. This item is a MUST.

Rod of Ages : Usually get this item if the enemy team is tanky and I know that I wont be able to kill them as fast. This gives you extra health and mana which

Rylai's Crystal Scepter: A popular choice for most melee APs. This item gives you a substantial amount of health and AP and of course, slows any enemy hit with spell damage (that includes the magic damage from your passive). This item is great for chasing, once you get a Crescent Strike off, your enemy will not be getting away from you. In addition, this item makes your Q a long range slow and your W a short range AoE slow so your escape capability is vastly improved with this item.

Zhoyna's Hourglass: A must-have for nearly all APs. Not only does this item give you a whopping 100 AP and a good amount of armor but it also can put you in a stasis for 2.5 seconds, which is by far the most powerful defensive spell Diana has. This is your ONLY anti-focus utility and because of that you should always make sure this item is off CD before you dive into a team fight. TIP: Always use the active on this Item just as you start to get focused, before you really start to take damage. You dont want to wait until your at 5% health and then use the item, you will more than likely instantly die after. You want to try and catch them at the beginning of their burst, so they have to switch targets and your still healthy enough to fight unhindered after the invulnerability is over.

Lichbane: This is this item that really seals the deal. With your high AP, lichbane gives Diana the potential to autocombo. At max build, every time you use a spell your going to hit the enemy for an extra 500 damage. This item is an absolutely necessity for damage later in the game.

Void Staff: Unlike other MP items, void staff lets you ignore a % of the enemies MR. This is crucial late game when you might need to kill enemies with a lot of MR. This %-based reduction really lets you deal the damage you need to deal, even if the enemy has a good amount of MR.

When I Play Diana I begin my poking as much as possible using Q. As soon as their AP / AD Carry falls bellow half health I signal to my team for initiation. Q one last time followed by W and then R. Land a few auto attacks till Q is off cooldown and then R again. By that time ACE.

For masteries I like to go 21/9/0 to maximize damage as assassin as well as the extra health early game.
Jungle

Lane

Jungle Route
Blue ( requires help from mid and top ) > Wolves > Wraiths > Golems > Gank > Red ( you can give to ADC ) > Repeat

Okay, now here is farming. At the start, you will have your Crescent slash, so don't be entirely dependant that you will receive max farm. But you can farm easily utilizing her passive, which helps a lot with farming. Then, as you level up, you will get Pale Cascade (Hopefully you would if you use this guide :P) and Moonfall, and so you can Crescent Slash to damage, pop your shield (Pale Cascade) and Moonfall, and that will be a whole minion wave destroyed, so Diana can farm really well if your heart so desires it. I personally dont worry about farm, as i go for kills :P.

Here is my match history following this build

If you guys have any comments or constructive criticism please let me know!