Diana is a very versatile champion. I prefer her as a mid lane AP carry.
This guide is a work in progress, please forgive the lack of detail and art as a necessary sacrifice for expedience.

Diana is unique, because she is a melee nuke with a massive gapcloser. In a way, she is similar to Katarina in gameplay, but has some cc, and massive nuking damage rather than Damage over Time (DoT).
The best combo to use with her is Q-R-W-R-E
This does some nuke damage and gapcloses, sets her up with a shield and more dps, jumps back on the fleeing opponent or just deals another ult of damage, and draws the escaping enemy back to you with the E.
This works well for both builds, the Nuke placing more emphasis on the q and ult, and the Sustain DPS more about the gapclosing and the slow/pull (E) and shield (w).

Note: With the nuke build, Diana must be played more like an assassin or a more squishy AP carry like Katarina.

I hope the build guide and gameplay advice help! Feel free to give me some build tips/ideas. This guide is just for people wanting to try Diana out and help them get the hang of her without getting lost in the basics. I really enjoy playing as Diana and hope you all do too!!!

Guide will be updated and filled in with time! Thank you all for your patience!
~The Legionnaire~