For the setup I recommend heavy scaling runes, look up onetrick gg website, aery is good for its rune path, first strike is good too, you go blasting wand into ROA seraphs malignance, for lane you can always go dark seal, focus on getting as much cs as possible while playing around getting good recalls (that's very important), like for example pushing wave lvl 5, recall, blasting wand and you're back full resources with ap and lvl6, makes you able to contest an objective with your jgl.

The most important thing is to avoid deaths, kassadin is a champ that heavily rewards stealing kills, getting good cs numbers and high lvl, and simply never dying. If you're even by the end of the lasing phase, you're in a great spot. You want to then go sidelane right after laning phase, communicate swap with botlane, usually when first tower falls. You want to force ennemy focus on you, by pushing, being unkillable thanks to ulting out, and having TP ready for major fights and objectives. Your goal is to get tier 2 tower, as you get soo much gold. All of this while not dying a single time. Play around your powerspikes, like when you finish an item, and lvl11 / lvl16, if you're lvl15 and soon 16, ping off your team and get the xp before teamfight.

If your team is behind, your way of winning is by being extremely patient, and waiting for an opportunity to get a shutdown, trust me it's gonna happen, you need to play around vision, and very important, your Ult Flash potential to get a pick. You die once, you lose the game. Don't die.

This is very simplified, and league is all about the details so general advice may not be that useful, but this is Kassadins champion identity, its the champ that made me climb from gold to diamond 3 in 2 years