I might update this guide but its pretty straight forward even if you don't already know how to play Diana. You are a good damage dealer but you also provide initiations and tankiness for the team. That being said, you can actually die pretty easily especially without items, so be careful.

Pros:
Tanky
Low cd ult
Nice design(fun)

Cons:
Not much escape (situational)
Mana dependent (without blue)
May steal kills from carries (too fun to ignore a free kill)

BLUE--> WOLVES--> WRAITHS--> RED--> GANK
Ask a lane to help if you like but its not necessary. (there is no leashing anymore btw)

1.Always try gank lanes that are pushed.

2.Sometimes its not as important to kill the enemy as it is to burn their flash, force them to B or deny them from creeping for an extended period of time.

3.Don't ever tower dive unless all of these are true
a.You are 100% sure you can get the kill in 3 or less tower hits
b.The enemy does not have a stun ready
c.Their Jungler is on the other side of the map
d.There are currently no enemies missing from their lanes

4. When ganking, start by landing your Q, then W then E when they are in range.

5. When ganking (post lvl 6) run in with W then E when they are in range and Q just as they get pulled to ensure you land it, then R.