Alright so here is my Version 0.1 Guide for Kassadin. This champ is a beast if played right but of course, it requires a lot of good timed moves to be successful. If you are a player that seems to die a lot, it's maybe not a champion for you, but I hope that you will try him at the very least.
I've posted what I master in below, so you can check that one out for mastery selection before you get into a game.
Lvl 1: Null Sphere
Lvl 2: Force Pulse
Lvl 3: Force Pulse
Lvl 4: Null Sphere
Lvl 5: Nether Blade
Lvl 6: Riftwalk (of course)
Well, for the first part of the game, Kass is a very, very weak player. I can barely manage to get kills in this part and it's very easy to get ganked, so watch yourself. You're going to want to run in a back and forth pattern toward the enemy and try to get them JUST BARELY within your null sphere range, hit them, and back down immediately. Do this whenever the CD is up. If your force pulse comes up, try to get in with another teammate and hit them with the edges. You should be able to get away fairly easily since it has a slow effect. Keep your distance during this and don't get too cocky. It will pay off later game.
At this point, you should be able to do some decent damage, keeping in mind that you want to be a hit and run player. If you see a teammate in a battle with an enemy at less than half health, you might want to think about jumping in for the Kill/Assist, assuming they won't kill them by themselves. Riftwalk in(trying to hit them, if not possible, get as close as possible), then slow with your Force Pulse spell. Hit them with null sphere and wait for recharge. Hopefully you will have almost killed them by now or your partner will be able to finish them off. If you are close enough for melee, you can auto attack with Nether Blade to negate some of their armor.
You should mostly be jumping in on team battles and getting the ganks on solo enemies by now. If you went AP build, your attacks are going to do RIDICULOUS damage to them and they will fear you. Just make sure you aren't too close to a horde of enemy champs, or you will be mincemeat still. Kass is very strong late game so if you've made it here with a couple of kills and little to no deaths, you will be beasting the enemy.
*Will add more later on, time for a game*