Hey this is the first time I've ever made a LOL Guide so please understand that it might not be the best.
This Guide is all about Dianna which has been called just another Akali but with mana, but if you have seen her before in action you realize this is very different. she can quickly jump onto a target using her ult, she can then group pull targets going to escape her range as well as even slowing them, for survivability she has a shield which can becomes useful TWICE! making her able to absorb a fair portion of damage, lastly her unique angled skill shot is a great nuke and when combined with ult become a deadly combo

I take Basic AP runes really, I like having the little bit of spell pen., the armor against AD Carrys is also great, especially since i see Dianna's major roles as an Anti- AD carry. More AP with the glyph runes which scale more at higher levels so it gives you that bit more damage late game. Lastly the flat AP Quints which are good for early game damage.

Basic AP Masteries like my runes, summons Wrath for that increased ghost speed, increased more AP cause hey more AP, cool down reduction, more AP pen, more damage and more AP. the points reduced damage to minions as well as armor and magic resist and also damage to minions is to allow Dianna to stay as high on HP for those crucial early game ganks which can sometimes shape how the match will turn out

Boots of Speed + 3 health pots at level one in my opinion is the best for Dianna as it gives her that extra speed with ghost to get to the target and use her E to pull back and slow the target to allow you and you teams mates to get the kills.
Fiendish Codex second gives you AP, CDR and Mana regain which all are good for Dianna because it means more damage, less time waiting for abilities in fights and giving you better mana when you don't have blue buff which can become a problem.
Needlessly large rod is simply early game damage and Mercury Treads is just to give her some survivability.
Nasher's Tooth is great for Dianna as more AS means more passive hits, this combined with more AP, CDR and mana regain just makes it that icing on the cake.
Deathcap does this really need explaining? get it
Rageblade often cops me flack while in games as people ask why you using that you bad? why not its great for Dianna as it gives more AP and AS during fights with a little extra AD which makes you hit that little bit harder and gives u a slight edge if they are stacking MR.
Ryjals Crystal Scepter is more AP, survivability and a slow can't get any simpler.
Trinity Force, Sheen procs is great for extra damage, more AP, more AS, a chance to crit!, more health. great item on any class, very good on Dianna.

Basic Jungle skill sequence for Dianna but level E seconds for a shorted CD and more slow so they can't get away

Smites- makes the jungle ten times easier
Ghost- to catch up and chase enemies as well as a useful escape when combined with her Q and Ult

All and all I love this build as the burst damage is great, however she is vulnerable to being focused in a team fight but if you uses your skills in the right order and remember to aim for the carries you should be fine. Dianna is a fun Character but remember this Guide is not perhaps the best Guide out there but it its fun in my Opinion.