Hey!
Im Levi and I really love to play Sett. This champion gives me real boss vibes and thats how he got designed. First I played him Top Lane although I was a Support/Jungle main. I had some fun and did pretty good but after about 50 games it got pretty boring. Then I started playing my normal mains and didnt thought alot about this Sett pick except some fun games once or twice a month. Then I saw Targamas playing Sett Support and hard carrying the game. I thought that I need to try that and it actually worked. I started playing him a lot on support and now I thought I should do a guide for everyone that wants to learn this pick. Some more personal things about me:
- my name is Levi as I said before
- I am 20 years old
- usually was an Lee Sin one trick
- Started playing Support at the start of S11 as a Pyke/Bard main
- Sett connoisseur

-Knuckle Down-
Knuckle Down is Setts Q ability. For Sett support you want to use it for the Movement Speed part because Sett doesnt have a real gap closer besides this small part of his kit.
You can also use this very great to take down wards because its a AA reset.

-Haymaker-
Haymaker is Setts W ability. In lane you want to tank every damage you can to deal damage over the true damage part of this ability.If its done correctly you cant lose any trade in any lane. Later on you want to tank as much as possible after you used your W ability.

-Facebreaker-
Facebreaker is Setts E ability. There isnt much to say about this spell. In lane always use this spell with minions at the other side to stun the enemy.

-The Show Stopper-
The Show Stopper is Setts Ultimate Ability. Its your only gap closer so use it wisely. Its very easy to use if you play vs lots of tanks. Just take a tank and follow up with your E ability on the Carry to stun them. Dont be scared to tank a hook if you can use your R afterwards. If you dont play vs any tank try to "Insec" the Carry over a flank. Try to come behind the Carry and use this ability to pull the enemy Carry into your team

Sett Support is best vs. melee tank supports.He can tank every hook because of his W and can even reengage through his ultimate ability. As a Sett Support in lane tank EVERY damage so you can damage the AD Carry over your W ability. Sett hates long range lanes like Caitlyn or Ashe but he doesnt cares if their lane partner is any melee tank support. You shouldnt pick Sett into double range lanes because you ll get perma kited

You should build Force of Nature always if they have 3+ AP sources. This Item is great because it gives you lots of movement speed what Sett uses really good.

Frozen Heart is the best item against AA based champions. It is a little waste because you get no benefits from the mana part of this item so think twice if you want to build this.

Gargoyles Stoneplate is a very good item against true damage. You should build this againsnt stuff like Vayne or Camille that are permanently dealing true damage against you. But you shouldnt forget that you give a good Serpents Fang angle for the enemies if you build this item.

Redemption is nice if you always have long fights or chases. This item can change fights if you use it correctly. You should give it a try and see if you like it.

Knights Vow is a good item if you have 1 big carry that gets permanently one shotted in every fight. So if you need peel for 1 person build this item.

This item is very situational and I would be careful buying it as it is NOT really gold efficient as you dont get much out of the stats besides the active. If you have really good engage in your Team (e.g. Ornn + Jarvan IV) and you re only the follow up you can go this if you want.

Glacial Augment - Glacial is the perfect rune for Sett as he is a simple tank/engage support. Its nice to have an amazing slow after your engage with E. You shouldnt use your R to proc it because it pulls the enemy out of the glacial zone.

Hextech Flashtraption - Hextech Flashtraption is nice to have because the other options arent really good for Sett. Also you often need your flash to engage so you have a second flash to engage suprisingly over walls.

Future's Market - Futures Market is really great for Sett as he doesnt like it to much if hes behind. It doesnt matter to much if you early die because you have this rune.

Cosmic Insight - Cosmic Insight is great because you really need those stats.

Nimbus Cloak - Nimbus Cloak gives you extra movement speed and as you probably already know this is really great for Sett Support.

Waterwalking- Water Walking + Hextech
Flashtraption saves the roaming options for Sett as he has very bad roaming at all.