Not Updated For Current Season
you'll want to take the typical Mage Marks which will be Magic penetration. The more penetration you get in runes the more damage you'll do in game, even if they stack magic resist you will still do more damage then without them. Seals- I go flat ability power, i've seen other guides say mana regeneration flat and per/level, i don't like this only because Kassadin has a mana regeneration skill of his own, so you can take mana regen if you feel you're draining your mana too quickly, but i prefer the more damage. Glyphs- I take cooldown per level. If you look at it you'll have longer cooldowns in the beginning but you'll be able to dish out more damage with Items and spam skills later. Quintessences- take more magic penetration, to make sure your damage won't be off set by the magic resist some people get in runes.
at the start of the match pick up a mana crystal and a health potion, you'll have more mana to use Null Sphere and keep the enemy silenced and angry. When you get enough to buy tear of the goddess, head back to base, next buy boots of speed and keep up the pressure with null sphere and force pulse. with the passive effect on tear of the goddess you'll be gaining more mana on every spell cast which will make it easy to spam spells to get force pulse ready to use. Now you have a choice, at this point you have a choice, if you're doing well go ahead and get the soul stealer before finishing your sorcerer's shoes or if you're doing so so finish your boots for a better get away. Next item up is either the leviathan or finishing the archangel's staff, same as the boots if you're doing well get the leviathan for more hp stacking with your soul stealer if not finish your staff for more damage. If the game is still going on at this point, go ahead and make your lich bane and your zhonya's ring, thanks to the new sheen buff you can go ahead and make your sheen then make your ring, after that finish your lich bane.
I like to take 21/1/8 build, you will have less mana regeneration from the utility tree but you'll make up for it with damage from the offensive tree. don't take brute force there's more important masteries to get then brute force. Take the magic penetration and mage friendly masteries in offense. If you take heal then pick up the reduced cooldown for heal in defensive tree. In Utility pick up the increased base mana, hp/mana regen and 1 point into time spent dead. I like taking more into the offensive tree against the utility tree, you'll get more shear damage from offensive, the utility tree if for a little bit of cooldown and experience and some mana regeneration.
ALWAYS make sure before entering ANY fight that you have 4 stacks on your Force Pulse. The reason for this is if you are at 4 stacks rift in (x5) then Null Sphere (x6) then Force Pulse is ready to use. If you're going into a 1v1 fight make sure to first Rift walk in, for the surprise, then use Null Sphere to make sure they can't use any abilities just auto attacks, this will make even the most confident summoner to think twice about engaging you. They should be trying to run at this point, Use Force Pulse if has a great amount of slow on it and for a good duration as well. If they are not dead yet just Rinse and repeat, For team fights try and stay behind your tanks, when they have the attention use your rotation rift walk in, null sphere, force pulse then rift behind your tanks to make sure you are not targeted. And always remember you get stacks from allied spell casts as well as foe spell casts on your Force Pulse.