All you have to do with this build is not die, so play like you're scared before level 6 and use your skills (and autos where safe) to last hit and CS as well as possible. The better you farm early, the faster you'll get Rod, which means the faster you'll start scaling, which means the faster you'll do a lot of damage, which means the faster you'll get to insane wave clearing for farming and champion killing for "better farming" ;).

For levels 1-5, DO NOT use a single skill on the enemy laner, and just respect their zone while you farm as best as you can under tower since they'll probably shove you in since you're Kassadin. If you get ganked or dove, flash away and live because it's not worth dying and getting behind early and losing your matchup. Kassadin doesn't really have any hard matchups if you play with the right zone respect and safety of the enemy champion, but I will admit that him laning against AD mid laners is his hardest laning time.

Now that you have your 50 mana free flash, pressure your opposing laner. Use skills on him, while still last hitting with W and autos and using E to wave clear or harrass where possible. Pretty much every time your Q is off cooldown you want to use it on the enemy laner, the only thing about this is if against an AP harass heavy laner, then you would want to time you Q to be cast just before you receive harass as well so that way the spell shield blocks most of the return damage.

The biggest thing about this is that you're still laning, so you can still get ganked, so don't EVER use your ult to trade unless you can go for an all-in for a kill. Your basic trade combo is to slow with E, walk up while also casting Q, then try to get an enhanced auto in with W.

This is where you really start to ramp up. By now your Rod has a few stacks and you should have at least 2 or 2 and a half items so you have a bit of damage. You're going to want to roam with your team to other lanes for dives or for picks and get some free kills or objectives.

Now that you're going for kills you're going to be using your ult a lot. So your basic skill combo is R, E, W, Q for the fastest flow of abilities, or if you're close enough to cast them, you can Q, E, R, W.

You pretty much own the game, dive in and delete carries, then Zhonya's, then ult out and repeat to clean up the fight. Then just win the game. Ez pz.