When I first played Cassiopeia, I looked on Mobafire for some builds. Everyone knows you find at least a decent build for Cassiopeia which I didn't. So I played her over and over again to find how to build her correctly. IMO, this is how I think Cassio should be played. Also this is mainly a carry build, I will explain more of this later.

Pros:
+Excellent harasser
+Good passive skill
+Poison does tons of damage
+Does a good amount of damage (Late game can be a drag)
+A good carry if played right

Cons:
-Squishy
-Expensive

Early Gameplay:
Grab your two Health Potions and Sapphire Crystal and go mid or allocated lane. Put your skill in Noxious Blast and spam it. Literally keep your passive stacks up, which will make it so you can spam more. BUT, if you are mid be careful with what you spam, since you do not want your minion wave pushed towards their tower in case they gank. So what you do is time the minion wave's health with Noxious Blast so you can last hit at least 3 minions. So wait for their hp to lower and then hit them with Noxious Blast. If your opponent is pushing hard, let your team know so they can come for an early gank and let that idiot know who's boss. Obviously if one minion has low hp just auto attack for the LAST hit. I would say to go back either when you can purchase both Boots and your Tear of the Goddess. But if you're a boss stay until you can get both your Sorcerer's Shoes and Tear of the Goddess.

Midgame:
After more of your poison awesomeness and farming/being cool, yeah. Anyway when you hit around 6-9, this is when you should start ganking. If the other team is pushing mid, ask for help. Be aware of what is going around the map and if you need to gank/push top/bottom, do so. If your teammates aren't idiots, listen to them when they say "help" or "gank please." Follow the item sequence and use the recipe build for each item. When you are building Rabadon's Deathcap I would suggest buying the Needlessly Large Rod instead of the Blasting Wand, for obvious reason, more AP. I would suggest buying sight wards and placing them at bushes in mid lane at baron or dragon. Really it depends on what the other team is doing, you call it. If they have stealh champs, ask your support teammate to buy Oracle's Elixir . Also if you are having troubles and dying early, build into Rod of Ages or other AP/HP items, such as Rylai's Crystal Scepter. When you have time get the blue buff, before Teleporting back or just when you have pushed far enough.

Lategame:
While still buying items and building your skills properly, you will be bossing the lanes. At sometime between 11-15 you should have grouped up as a team and constantly be pushing lanes. Sometimes when you have Shen or someone who is fast, Teemo have them backdoor turrets while you and your team keep the other team busy. After a push or when you have killed off some of the enemy team, go get Baron instantly, but wait for everyone to be alive. Grab blue bluff when available. Teamwork is essential!

Deadly Cadence [Passive]

Her passive allows her to spam Noxious Blast and helps her in laning and keeps her from going back for mana. I would strongly suggest keeping the stacks up with Noxious Blast or any other abilities really. Since Noxious Blast cost the least amount of mana.

Noxious Blast [Q]

Early game you use this to keep Deadly Cadence up and use it to poison targets to use your Twin Fang. You can clear minion waves easy with this and harass early/late game. Keep them poisoned if they are stepping over the line! In team fights keep spamming this on the closest enemy or the squishy if you can. Remember you are squishy as well!

Miasma [W]

I personally use this for the slow when enemies are running away or when you are having a full on battle you can definitely just throw this down to stop them from running away earlier and add more poison damage. I wouldn't use this in combination with your twin fang. Again great for clearing minions waves and CC in team fights. Just remember, it's mainly for CC and the slow.

Twin Fang [E]

This is your main DPS output. Never, ever, ever, ever, use this without the target being poisoned, it's such a waste of mana and time due to the fact if the target is poisoned you basically spam this. OKAY, now I would strongly suggest using this with smart cast which on default is [ALT+E]. I use use T as the smart cast cause I hate pushing ALT every time. Now to set the smart cast just hit ESC and go to key bindings and scroll down, you'll find it. Put the T as the secondary button just to try it out. Now how it works is when you hit the smart cast, place your mouse over the enemy you want to target and spam it. Make sure they don't become un-poisoned by hitting Noxious Blast as it comes out of cool down and Miasma to both slow them down and poison them.

Petrifying Gaze [R/ULT]

As this does some nice damage I don't encourage just using this to do damage. Use this to either stun the champ and hit them with your abilities or slow down a runner. You can use Flash and Petrifying Gaze to position this so it faces in front of them. Since if you do this ultimate towards their backs it will merely slow them. So make sure you get the stun most of the time, or if someone with little health is running, either use Miasma or your Petrifying Gaze. Great in team fights for CC! You can also use this just to get someone with low health since it has decent damage.

Teleport

I love this cause I use this to tell me when to go back. Once I have enough for Boots and Tear of the Goddess I go back. But I never go back unless I die or when the CD has finished on it. When you want to gank and you have it free to use, it's a great tool. Port back and quickly grab items and then help with a gank or just support.

Flash

Since you're squishy, use this to escape or line your Petrifying Gaze or just use it to chase. You can Flash over small walls and is just handy in general. Something I love to do early game is tower dive to get the last hit and Flash away, they will rage! But you have to master that before trying it. So I suggest some custom games in your future.

Now I am not going to say much on why I used the ones I choose, but I'll try to explain. I used all AP items for the purpose of people who know how not to die easy. Like keeping back and last hitting and just surviving while still dealing damage. I do some AP\HP items but you can always build into more HP items or Armor/Magic Res if you feel the need to. If you're smart and play logically (Interm-Adv), then I suggest sticking to this items. If you are a beginner, maybe work with some suggested items or some survivability items before you use my item sequence. If you are soloing, grab SOME pots if they are harassing and remember to grab some sight wards for bushes/dragon/baron/buffs/etc. When you are level 18 grab all the elixirs, I love them! Always use the recipe build.

I build into offense and utility. The reason why only 9 in offense is because the rest are AD points and crit point which are useless to you. No point putting your points into defense since you're carry and want to put a big burst of damage on enemies. I also put my points into utility because of cool down reduction and since you are using both flash and teleport, my masteries are correctly placed. The neutral monster buff increase, mana regen, and base movement speed are all helpful while soloing or laning.

Thank for reading my build if you did and no troll votes/posts please! Leave some good criticism and enjoy!

4/21/11: Changed item sequence.
4/21/11: Added icons and colored text