Well, first of all you get Hextech Revolver on Diana for the spell vamp and 40+ AP. Mobility Boots because these are in my honest onion the best boots for any assassin or jungler. Sheen because it's amazing when you get those 3 stacks of and Q or Ult and hit them! Rabadon's Deathcap because it rabs, come on. Nashor's Tooth because you will love the CDR on her and it's just amazing and the attack speed helps get those stacks of her passive faster.

Lich Bane because it's a great speed boost an good for mid/late game vs Sheen. On to Rylai's Crystal Scepter for the AP HP and slow! It's really nice and I recommend it on any melee AP champ. Now, on to Zhonya's Hourglass, items like these really depend on what the other team is building. Don't get this if an AP mage is raping your team, that's just silly. NEVER FULLY BUILD WHAT A GUIDE SAYS BASE YOUR BUILD OFF OF WHAT THE ENEMY TEAM IS BUILDING! Also, you can build some more CDR (cool down reduction) if you want. But, blue buff exists so there is almost no point

Greater Glyph of Ability Power, Greater Quintessence of Ability Power, and Greater Mark of Magic Penetration for the AP! Scaling and flat ap is amazing on champs like Diana. The Greater Seal of Armor for just some flat armor. Simple stuff no real need for detail.

Pretty straight forward, you can to get last hits and bring a lot of the minions down. Later on when you have your you can use it then you can get your 3 stacks of to last hit tons of creeps. It can be fun seeing all of that gold rain from the sky!

With Diana's skills having effects on other skills (her causing her ult to not be on cool down) you should keep this in mind. Since her Is a curved skill shot if you use Smart Casting I would recommend taking it off of your Q with her for your first game as her so you can get the arch down. But any way. You want to start off by keeping track of her passive Moonsilver Blade, this give her on her 3d hit to do extra some base damage based on level PLUS (%60 of your AP) Magic damage. So you could time a good burst around your passive and .

She has a high skill plateau, meaning her ability's are mostly situation, like her pulls enemys in and you can use this for someone running away or if you are in a lane with someone like Rammus who can taunt you can pull them in for a taunt ect ect. So when playing Diana rememberDO NOT PLAY LIKE A MONGOLOID! thing about the here and now and manage your cool downs. So this means don't just rush in and Q,R,R and hope for a kill. time her cd's. Only Q,R,R if you know that will kill them!

Pros!
-Snowballs super hard.
-Great in the jungle.
-Her reveals pesky enemy's hiding in bushes.
-Is really good at getting kills on champs who run away.
-Super duper fun

Cons!
-Squishy.
-Her can be hard to get used to.
-Since she is new and people fear the unknown she will get focused in team fights.
-She costs 6300 IP.

Diana is a great assassin type of AP melee. Because of her ult she can dart in and out of fights. Her Q serves as a ward in a way in the sense that it reveals an enemy which is good for both lanes where people hide in bushes all game or to just cover the jungle faster. If you liked Akali you will love Diana and I recommend getting her.