Hi, I'm slaraka and I am a North American ADC main. An ADC main writing a guide about Cassiopeia for his first guide?! Cassiopeia plays very similar to an ADC which is a big part of why I like her, using my ADC mechanics to play Top and Mid lane as well. She is very mechanics intensive so be wary that you will not be playing her proficiently in a few games and if you don't like lots of clicking this champion may not be for you.
Despite being an ADC main I have probably played more Cass than any other champion (200 hours+)

Pros + Highest sustained damage output in the game. + Tank buster. + Mana and Health sustain + AoE engage with her ult + Speed steroid/ slows for escaping/ chasing + Great splitpusher/ duelist + Great at checking brushes

Despite her weak early game, once her sustain passives kick in she can stay in lane indefinitely and transition smoothly into the late game. Her DPS is unarguably the highest in the game, this makes her great for taking dragons when playing her mid and busting tanks top. Even when behind she can land 3-5 man stun with her ult+flash combo and allow the lanes who are doing better to wreak havoc. Due to her insane DPS and speed management she can outduel most champions and can escape from groups for great splitpushes. Her W grants brush vision and her Q gives you a speed boost only if it hits a champion (so you can confirm the location of a stealther.)

Cons - Weak early game. - Doesn't have much burst. - No built in dashes. - Bad at roaming compared to most mids. - Item dependent.

Your weak early game leaves you vulnerable to lane bullies with kill potential and early ganking junglers. Your skills focus on low cooldowns/ high DPS, you aren't an assassin and aren't good at picking off low health targets. Despite your speed steroid on your Q, you can only get over walls by flashing. Due to your lack of mobility, lack of burst, and the fact that you are primarily concerned with farming into the late game, there are MUCH better roamers if your team needs the mid lane to be there to bail out the other lanes (this is part of the reason I think Cass is better top than mid even though I play both). When Cass is behind she isn't getting enough items to really take advantage of her passive % AP steroid.

I prefer to play Cassiopeia top since her rework and find that she excels versus most of the tanks that play top (consult the threat assessment bar) and when your team can benefit from a splitpusher. Communicate with your team when going Cass top because if you go an AP top with an AP mid then you would need an AD bruiser jungle and a tank support, keep an eye on what your team needs!

I like to play Cassiopeia mid when the bottom lane can fend for themselves (if your bot lane has a stronger early game than theirs you can go Cass, if your team has something like Vayne/ Kog'Maw vs Caitlyn/ Draven beware!) When you are mid you aren't as concerned with splitpushing as you are with dragon control, the new dragon is HUGE. Beware of lane bullies mid lane as they can really wreck you (if your jungler is able to gank more than theirs it can compensate.)

Independent of if you are mid or top if the enemy team has an early ganking jungler that is something to be wary of picking Cassiopeia mid into ESPECIALLY if you are already laning vs a bully.

Early Lane

When laning with Cassiopeia you generally aren't going to be attempting to making flash/ kill plays. Versus most matchups you are focused on surviving, and landing your poison on them. Every time you land your Q you get 3 stacks of your passive (you also get a stack every 6 seconds, landing your poison just accelerates your stacking.)
You should be last hitting with your E when it is available for the mana back, do not be afraid to put your E on a full cooldown while laning (it only goes on .5 second CD when target is poisoned.) You can lay your miasma down and chain multiple mana refunds together off of it, but you must execute about 3+ to make up for the mana cost of the miasma, be careful to NOT push lane doing this!

Late Lane

Eventually sometime after you have both enough mana sustain from items AND 100 stacks for health back you can start to try to whittle down your opponent (versus champions you counter/ tanks this happens earlier). To do this you want to poison them and land exactly 2 twin fangs on them (try to land your W to slow them and then land an easy Q after that.) Keep trying to land your Qs on them and land an E on them every 5 seconds to keep the debuff on them (you can have your E go on cooldown at this time but it isn't optimal.) They will slowly lose either too much health to stay in lane or be forced to go all-in on you so be prepared to kite/ keep your distance depending on the matchup (you can't win most all-ins yet!)

Mid game

When you your mana items have stacked for awhile (this phase happens early if you went the tear route) you can afford to spam your E onto them to deal even more damage without running OoM easily. At this phase of the game depending on how laning went you can start winning all-ins instead of just poking, you still need to kite carefully because if you are put under hard CC you won't be able to spam your Es so no damage and no healing. From now on you must be careful to always land your Es on poisoned targets (250 passive stacks makes this easier, if you miss your poison don't cast E and wait for a poison to come back up.)

Late Game

At this stage of the game you have your core items (ROA or Seraphs + Sorcs, Voidstaff, Rabadons) and 500 stacks (you usually get 500 stacks a little bit after your core.) At this stage of the game you have ridiculous amounts of damage and healing. You can likely consistently kite around and poke into certain 1v3s (early game champions, groups without a whole lot of crowd control) if you are VERY careful when doing so. The only 1v1s to be scared about at this point are champions like Tryndamere and Veigar. Depending on what your team needs/ compositions you can choose to splitpush or teamfight.