As the headline says, simple guide for Diana. Pew pew moonbeam!.. oh, sorry.
Remember, Mid or solo Top!

For the items. You should start with a dorans ring or boots and hp pots. That's up to you. Always get a dorans ring either way. Then go for Nashors thooth based on the fact that her passive is insane. Then go for Sorecere's shoes. Then just get Sheen, Hextech Revolver. Rabadons Deathcap, Lich Bane, Hextech Gunblade. 6th if up to the situation. Do you need a Abyssal Scepter? Get one. Or Banshes Veil.

Q-W-E-Q-Q-R-Q-W-Q-W-R-W-W-W-E-R-E-E

When engaging a target. Start with landing a Q then use your R to get to them, instantly use your W do have them take all the orbs. Then procced with E and then Q and R and so on.
If you're talanted with the champion and timing, try to time your spells so that most of them are activated after 2 Normal attacks. So when you have your sheen/lich bane you get out the full potential damage.

Just go with Flash and Ignite. Or if the team needs it, get an Exhaust.