While some people say she is one of the hardest players to play as, she is really simple in what your are trying to do spell wise. Cassiopeia is definitaly a mid character, watch out for vlad when midding for his un damagable spell where he is really op because you cant poison him when hes in it.

Her passive is called deadly cadence because it allows her to spam spells better then any other champion expecially in early game. From level 1 your goal might be to harrass with your Q every chance you get while farming creeps with it, remember to time your poisons right to farm better. At level 2 you should be able to spam your E after poisening them with Q. By level 3, and on, your W should be your initiating spell, slowing them down, hitting with E if you can, and hitting with Q if poison is about to come off of your enemy, of course spamming your E every chance you get. By level 6 with your ult, you can try to pick off any champ you can, or in team fight try to get 3 or more to help your team a lot more then most champs can with their ult, of course trying to do it when they are looking at you, it mostly works at the beginning when they are all trying to initiate.

The magic pen and ap runes are obvious, and the mana reg is good for early game, by late game I never run out mana so its really unimportant. DO NOT get cooldown reduction runes with Cassiopeia, it is useless for her in runes and items. The cd you get with your masteries are by far enough, her E cd from poison is all you need, and getting ap and magic pen is worth 100x more then cd for her.

Item wise she is pretty simple. Do not get cd with her at all, it is useless. The only change in my item build you may consider is putting in a rylais earlier. Though she has a slow, and a speed, slowing people with her e can do a lot for chasing down people if you cant with your W.

She can be super simple or super complicated... Poison them then spam your fangs.. if you can do this you will win.

Pros-
Able to spam spells
Able to slow people
Able to deal a huge amount of damage

Cons-
Less CC then some people
Can be difficult
Not bursty at all