Very nice guide, I am working on my own right now! It's been an hour and I just finished the intro. These guide things are hard. Bravo on finishing!

"get in this order"
"get in this order"
"no subs"
"it's good for getting health down"

The spelling errors, plus all of these blunt directives make for a worse-off guide. Make them more reasonable and explain why you do what you do more in-depth. Everything built in League serves a purpose.

I'm by no means a pro player either, but I think I can still offer a couple of pointers. Everyone has their own play styles, but I want to present some of my thoughts with my successes as Diana.

When jungling, smite is a necessity, even if you feel you can go without it. It helps you secure buff steals when counter jungling, steal baron/dragon, and speeds up your jungle clear times by an immense amount early game. That gives you more ganking opportunities.

I used to consider Nashor's Tooth a situational item for Diana, as there are better AP items. Before the modification to Fiendish Codex, with it having its Mana Regen removed, I feel that it is less viable unless you're doing poorly and need an inexpensive catch up item. Diana's AP ratios are quite high, making her an ideal burst mage. Relying on the Passive procs of Moonsilver Blade can reduce your overall burst potential, and even though Diana has great chasing capabilities, she can't chase forever. Often times, I've seen kills walking away from me due to relying on that last passive that just didn't go off, even with the increased AS from Nashor's. Moonsilver Blade is a great part of Diana's kit, but you really shouldn't rely too much on it for your damage output.

For the mana regen issues, I'd consider trying a Morellonomicon some time. It's relatively cheap and builds from a Gp5 item (Kage's Pick). It offers a good chunk of AP, 12 Mp5, and 20% CDR to boot.

Rabadon's Deathcap now gives 25% Ap boost
Quite a few spelling mistakes, google chrome auto-correct will fix that
with the right items you won't get mana hungry mid-late game
getting Moonfall at lv 3 can save your life