Hello, Guys.
I am Nachtarios and this is my first build ever so be nice ^^
I'm german, thats why there are some mistakes with my english - sorry for that.
Have fun with this build, it works really well!

this build is mainly designed for teamfights:
with this build youre pretty tanky, so you can engage easily without dying.
I would not recomment this in low elo or in soloQ because youre not as tanky as a tank - obviously. There are some nice combinations like: Diana + Orianna: Diana's E sticks the enemie team together so oriannas ultimate can hit all of them pretty easily. Be creative with that, you can make some nice teamplays with her E.
Please read the next thread in order to know how to act in teamfights.

in teamfights you always spam your q until the point, when you all of them (or many of them) are near to each other. then you ult in, press W,E and then your team needs to kill them. This build is awesome with premades, youre talking with. In low elo or soloq it often happens, that you engage, but nobody follows up. So be aware of that.

1. obviously AP for more damage
2. health in order to stay alive longer
3. magic resist, to do better in your lane because most of the midlaners are AP based (if not, take armor here)

skill W first against champs with an obvious lvl 1 spell you need to block
(for example: Zed's Q; Ahri's Q)
skill Q first if you think you can either trade on level 1-3(fizz; kassadin) or if you think you will get dominated in order to farm from behind (yasuo; orianna)

obviously, always take flash.
for the second spell, you can decide between ignite, (heal) or teleport.

IGNITE: when youre familiar with the champion. you need to get kills. roam a lot, and get your kills. be confident, if you take ignite. + you can play more aggresive

TELEPORT: i'd recomment teleport, if you play diana for the first time. (look for some nice teleports in bot or toplane to get some kills)

HEAL: take heal against enemies with ignite. it sounds weird but it works. Nullify their ignite with your heal.

farm whenever you can, its really important, because with the full build youre amazingly strong. Just trust me and farm as long as you dont need to roam. Again, teleport will fit this playstyle really good.

Your ultimate can be used to jump to minions in order to disengage.
You can Q in some camps to get vision and mabye jump over it with your ultimate.
You can also ward and ult directly but remeber - your ult will be on cooldown.
With diana you can nicely clean the raptor camp when you pushed your lane or you cant farm creeps because youre gettin dominated.
Your 3rd auto deals extra damage so I often do 2 autos on minions and then Q my enemie, jump to him with my ultimate and do my 3rd auto to get a little bit of poke. When hes low you can maybe use your ultimate again and kill him, but I mainly do that to harras him.

Thunderlords for more damage - strength of the ages for tankyness.