Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This build is to max out your Shunpo and Death Lotus damage, while offering good survivability.
The items in this build are the ones I use the most, but in no way are they set in stone in the order or whether or not I get them at all. While playing Kat you have to be flexible and aggressive. The items are suggestions and the only ones I see as a must have is the Sorcerer's Shoes and Rylai Staff. All the others depend on who you are playing against and how you are doing in the game. If you get fed early and instead of getting the Rylai staff right after Sorc Shoes you might want to get Mejai's in order to take advantage of early feeders before their team mates come to the rescue or they start playing smarter. If you are against two heavy physical damage champions, you might want to grab the Randuin's so you can stay alive long enough to get a kill or two. I'll go more into this later, but for the most part it is up to you as a player to assess a situation properly and figure out which items fit better in each situation.
Metagame: the masteries are built around providing as much damage as they can while giving some ability to survive. Runes give the needed magic pen in order max out Kat's abilities. They also provide very good health regen at high levels, and the cooldown reduction runes are a must for Kat since her damage is her cooldowns. An extra 10% cooldown reduction is pretty much a 10% damage increase for Kat so this is why you want to make sure you have enough cooldown reduction without giving up power or survivability in other places.
Start the game with Boots of Speed and 2 Health Potions. The potions will let you stay in the lane longer racking up exp and let you recover faster if you are getting harass too much or after a fight with another champion. Take Shunpo first because the single target damage gets higher faster than BB and because you want to max shunpo quickly to get the most out of your AP and gank better. It is also a good escape mechanism. BB should be taken on 2nd lvl and you should use it to last hit minions and harass other champions. KI + BB works really well against champions that have rely on health recovery(Vlad, Trynd, etc). KI + Shunpo should be used if you are picking a fight or trying to last hit something that might put you in a bad position. Also if something goes wrong with your Shunpo positioning, roll with it if you can.
This plays into being aggressive early game. Most champions with run away if you shunpo to them and hit them with a BB even if they don't have a reason to. Very few players are willing to do a stand up fight 1v1 before lvl 6. Most see it as reckless, but you should take advantage of it and throw them off their game. If you are shunpo-ing to them and getting in a few blows or using shunpo and then running away they won't know what to expect. Most players also feel safe behind a tower, but if your creep wave has moved up to attack the tower don't be scared to tower dive for a kill if you have more than half health. Kat's passive makes it very easy to jump in get a kill and jump out. If you think you can KI + Shunpo a champion and kill them with BB or an auto attack or two and shunpo out on your minions you should do it. It pays off for me about 60% of the time. This goes into keeping your opponent off balance by being aggressive and borderline reckless. If they don't know what to expect from you then you are doing your job as a Kat. The primary downfall to playing this aggressive is if your teammates won't back you up on it. I have run into times where solo queueing put me with overly defensive players that didn't want to take any risks. You will end up looking like a noob to them when you jump in to harass an enemy and they just stand there not doing anything to help.
If you can, try and take the solo lane. AD Kat makes a better solo laner due to her farming abilities with BB, but she can be very competitive either way.
To be continued...