So this is my first guide and might seem a little confusing, but I hope you get the basic ideas.
It's very short since I'm not a wizard. But I think this helps enough. I don't have potions or doran's stuff since they are expensive and slow you from getting your real build

First start to build Rod of Ages . Start up with Ruby Crystal and try to stay on lane until you get Catalyst the Protector catalyst the protector . Then you obviously follow up with Blasting wand .
And soon you're done and you have your stick of ages. Now you'll start building Nashor's tooth x2. First you'll get Fiendish Codex and follow up with stinger . After that you just get Nashor's tooth and repeat. Obviously you are going to need some AP so start building Rabadon's deathcap . If you can start with the rodneedlessly large, if not then with blasting wand . Rest is up to you but I recommend getting Rylai's Crystal Scepter and Lich Bane .

You simply take one third attack speed runes for Dianas passive and rest are non-flat AP runes.
Nothing tricky there

I chose 26/1/3 for masteries for quick kills and long range harassment with Q crescent and spells are simply for goodness of them. But of course it's not that strict and you can take flash over other one:P