Hi,i'm no high elo but this is my way to play kassadin,and it always works. This build will make you turn into a badass assassin that can 3 shot any non tank character.

For Runes i use ability power glyphs,seals and quintessences and magic pen for marks. This rune set will make your laning pretty good and your damage output will be awesome.

Masteries are pretty obvious..no need to explain this,just magic pen,cdr,ability power per level and utility.

Flash and Ignite, 2 spells that will help you a lot during the game,if you want you can trade ignite for Cleanse or Clarity.

Ok,farming is VERY IMPORTANT to kassadin,very very very important,and jungling,make sure to take blue buff everytime it's up.