Okay, you now know the basics, but to play this botlane efficiently, you need to know how to rotate the CC's well enough.
First of all, be sure to play this botlane with a premade member in which you trust, it is important to do that in order to engage efficiently.

The way to play this is quite simple. Both Sett and Swain have their E as their CC, and on this botlane specifically, Swain's infamous Nevermove is going to be your bread and butter.
What you want to do is try to land Swain's Nevermove when the minion's waves clash, if you land it, Sett will be able to get close with his Knuckle Down and stun with his Facebreaker, after that, go bonkers. It should be enough to at least do a huge chunk of health to anyone in the early game. the strong point of this synergy is that you can do it constantly as longs as your abilities are up, so don't be afraid to play VERY aggressively.

As with the defensive measures, you want to make sure to always keep Sett in front, because he's a big boy who can defend himself even if he gets caught, he could even turn around if he manages to stun both opponents with a Facebreaker.
Sett has a natural sustain with his passive, Swain however has to make sure to poke opponents frequently to stack his own passive, the more you do it, the better.

When the laning phase ends, exploit your lead to the maximum by constantly forcing risky fights for the enemy, as Swain and Sett have huge teamfight potential.

That's all folks !
Now go out there and make sure to make your enemies understand they're on the wrong side of history.