Sure thing!
Masteries: I go 21/0/9, making sure to take the mana masteries in support, and spellblade in the attack tree.
**Item build: **Tear/Seraphs, RoA, Lich Bane, Zhonyas, Void staff.
Skill max: Put some points into Q while in lane, after 6 max W, as it is where most of his damage lies.
Matchups to avoid: Heimerdinger, Ziggs, Yasuo, Marksmen or AD brawlers who can chase you. That's 'Nam flashback material, right there.
Basic theory in lane: Kassadin excels at harassing mages early on in lane due to his shield, and is now one of the most mana efficient champions in the game due to his W. Poke the enemy and clean up ranged cs with your Q, while last hitting with your W early on to keep your mana up.
While you may trade in lane, Kassadin's actual kill potential is very low until he hits 6. At most, expect to push you opponent away from his or her farm, and use your ranged poke to dominate your lane. Once you hit 6, try to bait out your opponent's cc and close, with the intent to land your W. Do not underestimate your auto-attacks, as they can finish off a winded opponent.
Basic theory out of lane: Once Kassadin hits 6, his kill potential and roaming potential skyrocket. Look for opportunities to gank other lanes, and ward deeply in order to catch the enemy jungler off-guard. Try to make the most of openings in other lanes, especially botlane to clean up and secure objectives, such as dragon.
During Teamfights: Kassadin actually becomes quite a bruiser come lategame. Play him as you would before, jumping in and out of fights to burst down enemies and draw aggro, before rift-walking to safety.
Good things to know:
Kassadin is one of the most mana efficient mages in the game now, and really dosn't need blue for it's regenerative properties. If you have a jungler that would benefit from the extra mana, let him have it.
Make sure to keep an eye on your Riftwalk meter. Your final build should give you about 3K mana, but 1200 a pop is no joke.
Kassadin is a late-game hypercarry with a high early, low mid and high lategame curve (like Tristana). He falls off a bit midgame, so be careful.
His slow is actually quite a handy bit of cc once you get used to it. Try engaging with champions like Nocturne and Lee Sin as a duo, slowing their target to allow them to close much easier. Lategame, Kassadin is beefy enough that you won't simply explode in one hit.