This is my first guide on MOBAfire. Any and all feedback is appreciated, so long as it is constructive.

To begin, Diana is a very bursty AP. She can be built to compliment your team, as well as sniping enemy champs over walls, she can jungle, yada yada yada. But the main thing we want to focus on is how she does in mid lane.

She's INCREDIBLY bursty. I've seen her 2 shot a Morgana then 1 shot the Ezreal standing behind her.
She has a passive attack speed bonus and every third attack hits like a damn freight train.
Incredibly fun to watch the squishies melt.
Come on! SHE'S A LUNAR CHAMPION! THE FUN WILL BE DOUBLED! Play Video

Almost useless unless she gets at least some sort of feed early on.
Her alternative skin makes her look like a night elf... >.>
Bad decision making on your part (I.e, pulling in the entire enemy team) WILL f@#k you over. No doubt about it.
She is also focused in team fights a lot. CC reduction is a good thing, but if you want max damage, go ahead.

Runes are standard AP carry. 9 Marks of Insight, Seals of Resilience, Glyphs of Potency. Then the standard Potency Quints. Simple enough. Nothing really to say here.So, to ruin your day, here's a pony.

Items, like any champion, are really what this boils down to. To start, I go Doran's Ring, which provides great sustain in lane with the mana regen. Of course, another viable option is to go boots 3 health, or 2 health one mana, but I've found that Diana is highly mobile, and by the time anything serious actually does happen, you'll have Sorcerer's shoes by then. Be sure to farm up enough to either get those t2 boots on your first trip back, or t1 boots and another ring. Once you're back in lane, you should be outpushing the other champ if you're properly landing your Q. This allows for easier farm for you since they cannot get close enough for fear of getting bursted down.

Hextech Revolver should be bought next time on your trip to the shop. This can be specced into two things; WoTA or Gunblade. Personally, I prefer WoTA far more because of Diana's passize. with that, every three attacks, you'll be getting a ton of health back anyways.

Rabbadon's, self explanatory. Free 30% AP. WOOHOO! Hit ALL THE CHAMPS HARD!

Rylai's gives you even more sustain while giving fantastic utility and AP. Obvious choice.

More spell pen? Sure! With this, you'll be hitting even harder than you were before. A nice choice, but is it needed? No. You could go with a Zhonya's instead. However, this does help quite a bit in any situation.

Lich Bane on Diana is kinda meh to be honest. For those who think her mana is just too low, think again. With those double Doran's Rings you got earlier, you'll have an easy time farming and spamming abilities. And plus, your jungler should be giving you blue. Aside from the last two items, that is the core build that I've found best for Diana. Be sure to build to your team's needs and not just your own, though.