Hello there.This is my first build :D.In this build ill show u how to play Cass.She can own everytime.And as time goes she become's more effective to the team.

For runes i take Greater Mark of Insight,Greater Seal of Clarity,Greater glyph of Clarity and Greater Quintessence of Potency
-Greater Mark of Insight is usefull not only just in team fights,but in 1v1's because her spells (poisons) ignore alot of MS and its usefull to have that to deal ton's of DMG
-Greater Seal of Clarity and Greater Glyph of Clarity are usefull for most of DPS/AP's.Not only at begining but at late game you got lots of mana regen and u dont need to go back that often because Cassiopia has a lousy mana regen and she needs it.
-Greater Quintessence of Potency is usefull for early and mid game as it gives ap and improves damage to all spells.

My masteries are 9/0/21.In offence u take the standard AP masteries for dmg and cooldown redution.In utility you take masteries that help's you with regen and cooldowns.Plus the masterie Awarness is usefull cuz you get advantage in XP wherever you are,in mid lane or at bot/top lane.

Just follow my item making and ill explain why i take these items.First of all Doran's Ring is your first and basic item.ALLWAYS BUY DORAN'S RING AS FIRST ITEMThe next item you make is sorcress boots.Alot of ppl asked me why sorcress boots?Why not Ionian for CDR?Well here is why....Cassiopia has got low cooldowns on her spells already so u dont need CDR items.Then the next item you rush will always beRyjal's Crystal Specter.Even if ure score is like 10/0.The Ryjal should be rushed and got at/before the 20th minute.The next item is Rabadon's.It gives alot of ap wich will increase your damage threw out the game and it is very usefull.When you get Rabadon's you should deal about 300 and more damage with first spell.The next item is RoA (Rod of Aegis).It gives HP/MP/AP wich Cass needs.After RoA you get Abbysal specter.It gives stats that Cass needs AP/MR (MR isnt that usefull but if ure enemy's are AP/DPS like Lux,Kassadin or Leblanc its usefull).And finaly sell Doran's ring and get Zhonya's Hourglass it gives AP and the special effect to be invunrible.Well like i said if u fight vs a strong DPS u use the effect of Zhonya's and defend yourself (example Karthus)

This skill Sequence is very importaint so you deal alot of dmg threw the early and the late game.
Noxious Blast is like Karthus's Lay waste.You use ure cursor to cast the spell.Now....That spell activates very fast and deals lot's of dmg plus it had a very low CD.And if you hit the target sucsesfully you get speed boost witch is a good combo with Ryjal's if you need to run/catch a enemy.
Miasma is usefull for its slow NOT! damage.You take Miasma at level 4 and leave it there cuz it deals very low dmg.But it has a slowing AOE effect wich stack's with Ryjal's and u can use it like Noxious Blast to catch a enemy or escape from one.At late game it is usefull for its damage,because if u play 1v1 againts anybody u use it first and fast to deal dmg and slow then spam other spells and deal ton's of damage
Twin Fang is the most Op'est spell at Cassiopia.It deals very good damage at early/late game(depends on playing)Plus if Twin Fang hits a poisoned target the CD is set to 0.5 sec wich is usefull for 1v1.Thats why Cass can be a good duelist vs DPS'ers (not tanks cuz they can stack MR).Twin Fang's effect to be set to 0.5 sec applies with other poison's so u can lane sometime's with Teemo and spam Twin Fang like crazy for crazy damage.
Petrifying Gaze is awsome in team fight's and in 1v1's.It deals damage and it stuns/slows.If u play a 1v1 vs some 1 who dies fast you dont need to use it.Only use ultimate if u feel threatened(if u have low HP).But if see that you got very low HP and cant kill the target dont use ulti.Use other spells to slow them and run away.

IGINITE
-Ignite is the 1 of the main summoner spells for Cass.You use it at battle's or if ure target is fleeing with low HP u can use it to kill him with it and the poison.
FLASH
-Flash is the 1 of the main summoner spells for Cass.It is always usefull.Escaping ganks,catching kills,saving ureself,saving an ally....etc.But dont use flash lightly if you solo a lane with her and the enemy has a jungler.Save flash for escaping purpous's.
CLARITY
-Clarity is a viable summoner spell for Cassiopia.Like i said you got mana regen runes but some players spam spells and they can use Clarty for faster mana gaining rather then waiting to get mana from runes.But still i dont use clarity cuz you shouldnt spam spells if u arnt in a battle.
TELEPORT
-This is the useable sometimes.If you start badly at begiing you should always try to stay in lane for XP.If you do something hasty and almost wind up dead....go to base!.But since you got teleport use it to defend the lane or get XP/gold faster.
CLAIRVOYANCE
-Clairvoyance is hardly used by a Cassiopeia.But its little viable.If you are solo with Cass vs one or two use it to scout arround and play defensive with it.If the enemy got a jungler and ure ally (jungler/ganker) needs it use it to help him.

Cassiopeia ganks very good.If you find a enemy jungler at the blue buff and he didnt pull the golem in the bush you always should wait in the bush while he kills it.Then you engage 1v1.With the poison's you hardly can lose.
At a low level Cassiopeia can gank but she deals low dmg.
At a bigger level Cassiopeia ganks very good with ton's of dmg.

When Cassiopia is mid or soloing a lane she always should wait to auto attack and last hit minion's.As long as you got a farmed Cass you got a happy Cass :D.Then you get alot gold.Sometimes if u got alot of creeps in 1 place use ure AOE spells to farm them.
In late game the second spell Miasma can kill a minion wave and its usefull for lategame farm.

In large team fight's where it is (near enemy or ally base,woods,baron....etc.)you should always wait for your ally's to engage never engage yourself.
In those teamfights sometimes its ure job to fastly dmg a carry so u get rid of the enemy's DPS(example Caitlyn).Thats why you never engage.If you do engage and all ure enemy's are in the direction of ure hero(looking at Cass)use ure ulti to CC the whole team. If u ever find youself in a bush and the enemy heroes are coming there with low hp or to recall dont use ure ulti!
If you want your team fights to be sucsesfull always have 1 engager (example Katarina,Garen,Lee Sin).

Here is a importaint tip:If you got an OP enemy team (example they got alot of AD/DPS)buy a thornmail.It depednds on enemy's and what item you want to buy.You can also buy a Guradian Angel.The most importiant thing to know is the more time you are alive you have the most benefit to your team.

I hope you use this build :D.Accually and honestly its my first.I used this build myself in lots of games(i made this build up).
I hope you wont be dissapointed and enjoy playing Cassiopeia:D

P.S. I know i missspelled Cassiopeia alot of times.