Disclaimer: I'm_ alright_ as Riven. I'm by no means good at her. Just alright. Take what I say with a grain of salt, and be sure to watch Box Box, Viper or some other Riven main that knows more than me. If such a Riven main can confirm if I got this right, that would be appreciated.
Well, animation canceling is simple, in essence. Ability>M(move)>AA(Auto attack)>Repeat is the general idea. Once you know the concept, it really does come down to you practicing it. A decent fast combo would be Q>M>AA>W>M>AA>Q>M>AA>E>M>AA>Q>M>AA. Feel free to mix it up. For disengages, I suggest ending the combo with W>E away. This will stun, give you a shield and put some distance between you and your target.
Against Darius, test the waters and bait stuff out. You wanna try to get him to use his Q. After that, give fighting/poking him a go. Use your first two Qs passively on minions, toward the very edge of the range of his Q, and see what he does. If he bites the bait, hit him with your third Q, then W>E out. If not, then do the same thing on minions to try to get any low HP minions within his range, still trying to get him to blow his Q. If you space your Qs out enough, you should have your Q back up really really soon, so you'll be able to punish him a bit more or all in him if he made the mistake of biting the bait already.
Swain is a sustain tank (Sort of). You wanna watch what he builds here. If he's building AP, rush
and possibly even
or
if he's ahead. If he's building armor, go ahead and get your cleaver. Using your fast combo that you learned will help (in all matchups, but against sustain focused champs especially) because it will reduce the amount of time he has to get as much sustain as he can. Taking ignite for the grevious wounds is good too, if you're still in-confident.