First of all, Hello!!! This is a simple guide for all of those who want to try out cassipeia. She's great nuker, with haveing slow,movement speed increase and stun she can really stick to her target. One of the most important things is to learn aming, and to have the right timing.
I use Magic penatration reds, Yellow MP5(not per level cause from my expirience i have enough mana to spamm with rod of ages and her passive), AP per level blues and 3x FLAT AP quints. I found this really good rune setup tough you can change it to whatever you want, this is just simple idea of what you should use.
For masteries you I use simple caster 9/0/21 caster setup. Taking just magic penetration in offence!!!
Item build can be same for any kind of game. Only things that you need to watch out for is if they have a lot of mele take Hourglass, and if they have a lot of magic dmg take scepter.
First take Q and leave it there then put 1 point in E and then 1 point in W. After that level E and ulti whenever you can and after that level Q to max and then W last.
For summoner spells i go with Flash and ignite. If you are one of those who cant keep up with mana costs or you dont use mana regen runes you can consider taking clarity.
It is really easy to farm with cass. Just spamm Q and put W under the caster minnions, nothing special. Another thing if you can and if you'r jungler let you ALWAYS TAKE BLUE!!!!!it is really easy to kill golem. Just put W under him and spamm E!!!