NS = Null Sphere
NB = Nether Blade
FP = Force Pulse
RW = Riftwalk

This build is a lot like all the others. Runes and Mastery Tree are mostly the same as well. My spells are different however, I prefer to keep (clarity) early game the Mana is needed for Spamming. Ignite helps to get those runners too...even tho Kass can chase for days...you don't want to get to greedy.

To start off get Meki Pendant and 2 healing potions, your first skill should be Null Sphere. Use NS to farm creeps if you can't get close enough, again don't be greedy. Put your subsequent skill just like the guide says. I usually stay out till I have around 1100 gold then go buy Tears of the Goddess, Boots of Speed, and 2 more health potions if you used the ones you had.

Now its time to be a "LITTLE" more aggressive. By this point you should be at least level 6. Use NB to keep your FP charged up, and continue to farm creeps. Here how I harass, (This should all be done in a split second). :P
RW next to enemy champ.
(you don't have to hit them directly with this just get close enough).
NS so they cant cast on you/Dmg.
FP to slow/Dmg.
Then RUN away...before they can damage you.

Repeat the process until they are low enough. When they are low repeat the combo one more time but this time don't run. Cast Ignite and attack until they either are dead or have reached there tower. At this point hope your Ignite will kill them.

Kassadin doesn't have a lot of life or armor, Especially with this build. Hit and run tactics are your best asset, which is why I have clarity. Having the Lichbane in this build makes killing Champs, with magic resist, a breeze.

I hope this helps everyone. Just remember Good luck and Have fun, even if this build doesn't work for you.