This is my First guide ever. I'm not going to overwhelm myself , so it's going to be short and simple :D.

This Diana build is really successful and ,if you do really good, send me some screenshots of your game match history.
P.S. Diana shouldn't be played mid. (I've seen it a couple of times)

Dominion 18 Kills-5 Deaths-15 Assists (9 Captured - 3 Neutralized)

You can use whatever you would like for runes, just keep in mind she sorta needs the ap to thrive early game. I just carry the typical AP Mid runes on Diana.
Greater Mark of Magic Penetration, Greater Seal of Ability Power, Greater Glyph of Scaling Ability Power, Greater Quintessence of Ability Power
Ditto for masteries.
21/0/9

Berserker's is a must have if you want this build to work it's really good at giving you that 3rd hit for your passive and it compliments it so well.
I prefer to build this over Sorcerer's Shoes.

This item is PERFECT. Lowers magic resistance, so that when you're "Shredding your target" your passive will deal more damage. It also adds small magic damage to your physical attacks. Gives you attack speed and ability power Nuff' said.

This item will solve your Mana problems.
On top of that your cooldowns will be reduced and with a cherry on top, you get more attack speed and ability power. [By now your passive will do around 350 damage.(depending on your level)]

Rylai's is of course for more health and also for the 15% slow, Which will make you more of an assassin. Gives a lot of AP no attack speed though ;(.

Lich bane will give you more burst and movement speed which will make you even more of an assassin. If you mix up you abilities with your basic attacks(which you should) this item will get you kills. Remember that it you can use it only every 2 seconds so don't try to be a riven.

I've never tried it but sounds great (but keep in mind at this point your attack speed really doesn't need to be faster.):D.

This will increase your damage and if you have a Lich bane this will give you "tons of damage".

Get this item early on if you really need health. It will sacrafice some damage, but at least you will stay alive. I still prefer Rylai's Crystal Scepter over this item though.

Good item if the other team is building magic resist.

Haven't tried it but looking at the stats and looks like good sustain/damage.

Farming

E -> W -> shift+right click -> Done

Her passive will murder all the minions :D.

Combo

Q -> R -> E -> W -> R or Q -> R