Prepare your anus. This butthurt will flow more swiftly than anything you have ever felt before. Ap cait will blow your mind, blasting you with her deadly net and traps of death.

I choose standard runes of awesomeness when it comes to ap Cait.
Magic penetration marks - For extra net penetration
Mana regen per lvl seals - To get the sustain you need
Flat AP glyphs - So you have that little extra dmg early game
Flat AP Quints - So you can totally molest.. anyone

Masteries are important
Standard *** kicking ap build with 21 points in the offence tree.
Put the rest of those cheesy points in the defence tree so you get some extra survivabillity.

Ok you need to start out with your net. Go to your lane, either bot or mid. I prefer bot, because when you're gonna own with ap cait, you need those bushes.
Now you can decide wether you want to net people to death, or trap them. Either way owns.
Nothing is better than watching people walk onto those sweet traps and see their nice green healthbar vanish into another dimension.

Now what you need to do with items is start out with a dorans ring.
People won't be expecting ap cait to do much. So play passivly and last hit.
Since you're cait, you can pretty much own any bush you want, especially when you're ap cait. Those traps of death are just... like yeah, death. So throw them either out from the bushes or into. Keep your back against the wall and throw out deadly nets which will slow people down and hurt them badly. Yeah... they will also get slowed, so you're actually mind****ing them at the same time. Work for 2 dorans, lvl 2 boots and lucky pick for early game, then rush deathcap. This will allow your dmg to just own so much harder. After your cap is done, get a sheen and as soon as you see someone step on your trap, you blast them with a net and auto attack, sheen will proc and gg (an enemy has been slain). This will pretty much happen once every 20 seconds. Now you need to rush lich bane or deathfire grasp. I like lich bane for some sustained dmg as long as you kite well.

That's pretty much it.
And yes this guide is totally legit.