This build focuses on denying farm early game and taking out the tanks in late game
Cass is a very squishy champion so she should not be in the midst of team fights, but outside pocking at all the champs with her poison from a distance
Cass's Noxious Blast should always be used to poke at her opponents during the laneing phase, keeping them away from the minions, and the Miasma should be used to hit the mage minions as well as keep them at a distance away from the minions as to prevent them from long range last hitting.
The build is not extremely expensive so surviving on just farming should be fine for early game.

Usually masteries are used to increase the damage of mids, but with this type of play, sustain and increased gold are very important.

Early game Cass is very mana hungry, but if you only use Noxious Blast to poke and keep your opponent at bay, the Doran's ring should keep you afloat until you purchase your Chalice
After you reach about level 4, using the Miasma to farm becomes very efficient, and when at the opposing turret, using the Miasma to keep your opponent from last hitting the minions and poisoning the incoming minions will allow you to take the turret out much quicker
Be aware, that if you start farming off of the opposing turret, that the turret may start attacking you because of poison damage on your opponent
Also be aware of ganks, after level 6 your ult can help save you from ganks but early game it is extremely dangerous to push past mid area.