First I have to say: I made this guide for the additional tickets in the giveaway, I don't really know if it's a good guide nor do I care.
Secondly: I'm not a player that's actually accomplished anything in his league career. Although, I don't even play solo/duoQ I that I play clashes on 3rd/2nd tier and my performance isn't that bad there. That's all, I guess. If you have any questions about this guide, then just ask them in the comments. Most of the matchup knowledge/tips are from my own experience and guides from actually good players. This guide was checked on by my friend who achieved master tier on EUW, and he told me it's pretty good.

The most important thing people have to learn about Sett is that he's not a hypercarry 1v9. He's pretty much a support. Defend your ADC during teamfights or try to CC the enemy ADC to make the job of killing him easier for your teammates. Don't go in 1v5 for a montage unless opponents are 5 items behind you. Remember that during teamfights the ADC is the most vital part of your team, and you should always either prioritize defending your ADC or making the teamfight easier for him to pop off.

There are actually some tricks when it comes to Sett's very simple toolkit.

Cancel your AA animation using Q to maximize damage.

Position yourself so that you always stun opponents with your E.

You can also maximize your E's range by using it diagonally. (test it out on practice tool)

If the opponent doesn't have any dashes, or flash left then E W and R W is pretty much guaranteed.

If you hit a stun with your E, then your W is guaranteed.

Also, if you hit a stun with your E, you can AA,Q,AA,AA unless they have any dashes then it's guaranteed.

Your R always leaves you behind your target, use it to set up E stuns.

And that's all you need in order to dominate your opponents on top. Experience matchups, improve your wave management and map awareness, and you should be good.