-I'll post screenshots, if I ever get around to it. ;)
- I have a tendency to ramble. Deal with it. ;)
- Do let me know what else you'd like to see, but don't expect me to give you what you should do in every possible situation you could ever come across in this guide. Its a guide, not a play by play of your game. ;)
- Constructive Criticism is ALWAYS appreciated.
- If the skill sequence is funky, Sorry. I have trouble seeing the little boxes. :( If you're trying to follow my skill sequence exactly while your playing... you have more issues than any guide could ever help you with. Its a general guideline. :)

Anyways, now to the good part. Cass is one of the most underrated characters in the game, mostly because so many people suck with her. Shes really not a particularly tricky character, once you understand how she works. Just a lil' bit squishy.

Now.... If you don't read anything else here... CASS DOES MOST OF HER DAMAGE OVER TIME. CASS IS VERY SQUISHY. PLAN ACCORDINGLY.

Noxious Blast - This is one of the more difficult abilities to hit with, but gives a nice little speed boost if you do. Once you get the hang of it, however, you'll look like a badass with how often you hit. ;)
The trick to this ability is to remember that whereever you are facing, is where it will go. Once you hit the button, be careful you don't turn around to fast, because it isn't quite instant cast.
I find it difficult to use at the ends of its range, but rather easy to use on a stunned or fleeing enemy. It does a fair amount of damage (even though it is over time, like the majority of Cass's damage), and a fleeing enemy that got away? Well... they didn't. ;)

Miasma - This is a cloud of poison, very handy for killing minions (which tend to bunch up), as well as during teamfights when you can slow/hit several enemies with it. It's important to remember this ability does have a slight slow on it, so can be used as a (poor) escape mechanic, or more likely to prevent the other teams escape w/ a small slow and a little bit of damage.
Getting this ability first will allow Cass to reap havoc on minions early on, and an accurate Cass can easily harass with it as well.

Twin Fang - Twin Fang causes the most immediate damage of all of Sona's abilities, plus it has the nice little bonus where, if the target is poisoned, the cooldown is greatly reduced. Correct me if i'm wrong but.... This one seems self explanatory. ;)

Petrifying Gaze - This is Cass's ult. It is very similar to Sona's ult, in that it can be a mass stun. It will 'petrify' any enemies facing Cass, and is a nice slow on anyone facing a different direction. Timing is everything with this ability (just like the rest of her abilities... lol). A good stun for a good team WILL cause an ace. A good stun for a bad team will... well... let you get a kill in. A stun then laying down your poison is extremely effective.

While, obviously every game is different, in general the two most useful to Cass will be Flash and Teleport.
The reasons are pretty obvious... flash out of danger, teleport into a fight. :)

Starting Item: Meki Pendant

The rest of the Item order is designed to give Cass damage fast, as well as a very small amount of survivability. Still, remember she is squishy and this build is not designed to address this.This build is for a game/playstyle where Cass is not fighting 1v1 or 1v2. I have another build i've written that I'll publish eventually with a non-squishy Cass (Well... less squishy anyways). This is a pure AP build designed for maximum damage output, not intake...

Tear of the Goddess
Boots of Speed
Sorcerer's Shoes
Will of the Ancients
Archangel's Staff
Void Staff
Lich Bane
Rabadon's Deathcap

I think I've explained everything you need to understand how to play Cass but... here it all is again. :)
1. Combine your Ult with your poison effects. And your poison effects with other poison effects. ;) If you don't kill someone right away, don't panic. People tend to die after 'escaping' Cass. ;)
2. Remember, this isn't a build for a mid or solo lane Cass. I have one written, but I haven't published it yet. (As of the time of writing this)
3. If you suck at playing Cass, you're either not combining your abilities correctly, not hitting actually hitting anyone with Noxious Blast, or you have ignored everything I've said about what this particular build is for. :)

Alright, once again, I do not pretend like this is the best guide, or the worst you'll ever read. Feel free to comment with constructive criticism, and remember, everyones playstyle is different. What works for me may not work for you.
This guide obviously still has work to be done on it, but, i decided it's potentially helpful enough to be published. ;)
I hope you enjoyed reading!