Ignite is a summoner spell that I love on Diana, I mostly use it to clean up fights during early game, I don't use it much late as my items are good enough to finish them off anyway.

Flash is just a must-have for any champion, when I play Diana, I always get ganked if I push a little or overextend, flash lets me escape those sure death situations.

You can use other spells such as Exhaust for chasing, but you can chase with your ult, so I think that ignite and flash are the best ones.

Starting with the Crystalline Flask and three hp pots gives you so much sustain, you'll need that sustain to win against champions like Ahri, who has spell vamp on her passive.

Because of the sustain, i've destroyed a lot of turrets which have led us to win the game.

I always buy Seeker's Armguard after my first recall, it gives me a little bit of armor and AP, it's good to buy boots too, but I normally buy them during mid/late game so I can use my money on other items.

My first core item I buy is the Abyssal Mask, this gives me the magic resist and AP I need to win my lane. Next I rush a Rabadon's Deathcap, once you get this, you'll be doing soooo much damage with your Crescent Strike.

As I said earlier, now is the time I buy my boots, I personally prefer sorc boots, just for that extra magic pen., but cooldown boots are good too, as you can spam your abilities.

By now it'll be mid/late game, and you'll need some extra damage to win in teamfights, so a Void Staff is a great choice.

To top off all that damage, you can now build your Seeker's Armguard into a Zhonya's Hourglass, this is just a great item, both defensively and offensively.

Always max your Crescent Strike and Pale Cascade first, but take a point in Moonfall early as you will need it for chases. Remember, when all your orbs on your w explode, the shield is increased by 40%, and when enemies are hit by your Crescent Strike and have a shining light on them, if you hit them with you ultimate Lunar Rush, the cooldown is immediately refreshed, and you can immediately recast it, this is a great way to get some extra damage and catch up to enemies.

The 21 9 0 masteries are made to give you a ton of damage to dominate your lane, and the extra health and armor help, as your items only give you a little bit of armor.

The runes are placed to give you a good amount of magic damage, while also giving you armor to defend yourself against AD champions.

I am a lvl 17 noob who only played Diana because she was free, If you think that this guide is bad in anyway, tell me.
This is my match history: