Heyho! Former 2'nd ranked Ivern on EUNE, and about 20'th worldwide is here for the.. second time. My first guide on Ornn was done long-long-looong time ago when he was freshly released, so seeing that I'm doing decent-ish on Sylas, I have decided to share my few bits on him.
If you will find anything that you consider wrong, or simply don't agree with, feel free to leave a comment. Same if you feel like this guide was helpful in any way.

Everything is going nice. That pesky Teemo is about to die, and you - the mighty Riven - have prepared your best combo! Hooza! Time to kill him!

And then suddenly you get chained, your health is almost gone, but! You got out. All safe and nice. I am awakened. Ah what a lovely music to your ears, as you are about to slam Teemo with your ult, but then you realize! You didn't press your R! and before you know it, you are dead.

Yeah that happens, but in all honesty, Sylas is a decent champion. People don't know how to play him, and that's understandable, since even in mid-higher ELO you are not obliged to know how every ult works and what to expect from it. Hell, he was freshly released and few days after barely anyone bans him. That's enough of a proof.

His advantage comes off enemy champions, and that can be both his con as well as his pros. Even before the round start you need to know how you will utilize enemy ultimates, and how you can gain dominance even from earliest level. Being mediocre jungler, you have to utilize with Sylas everything you can.

With his lack of fast clear level 1, and rather weak clear against BAM*, you will need to take a bit of unusual route.

Blue Side #1

Red -> Wraiths -> Wolves -> Scutler/Gank -> Blue

Have a strong pull from bottom, level 2 you can swipe wraiths using smite, and after that you will have to wait anyway for your smite to come again to clear blue smoothly, so you can do wolves, then scutler or go directly to gank mid or top. This route may cause you to meet enemy jungler on scutler, so be aware of that. After gank or scutler, or both, you will have smite back up and you can clear blue without any issue.

Blue Side #2

Red -> Wraiths -> Scutler/Gank -> Blue

This route is preferred when enemy jungler is faster than you in clearing camps. You can expect him to come to your blue, but usually they won't risk it until their buffs are cleared, giving you enough time to grab scutler - so regain your lost on wraiths health.

Red Side #1

Blue -> Wolves -> Wraiths -> Scutler/Gank -> Red

Mirror route to Blue Side #1.

Red Side #2

Blue -> Wolves -> Scutler/Gank -> Red

Mirror route to Blue Side #2

Summary

You will use the scutler to regain health from previous camps, and/or go gank lane that will be open for it. That way you will have both smite and health to take on respective second buff and still be able to proceed with next possible gank, or finish off your jungle clear, before going back to base with enough gold for items.

Clearing Wraiths - Q to hit small ones, Strike, E, Strike, Recast E on big one, Strike. Finish off small ones before dealing with big.

Clearing Wolves - Q, Strike big, E, strike big, Recast E, strike big, finish off small, then big.

*BAM - Big *** Monster

So you want to gank a lane. You see that tasty Yasuo level two, pushing deep, deep into your allied tower. And you can see enemy jungler goofing around with scutler on the other side of river, too oblivious to realise what is even happening in the entire game, busy with that sweet blunt of his/hers.

.
.
.
Yes. It's time.

Of course your natural instinct will tell you to slam that E hard, hook enemy fast and shatter him down with prominent Q! And technically you are right. If the enemy wastes his escape mechanism in combat, well, that would be stupid not to abuse it.
On the other hand though, it's overall best to start the fight by engaging with Q to slow the enemy down, casting E to dodge any stun or root, and then recasting E to stun the enemy and let your ally close the distance. And if you are against someone with heavy dash, it's time to beg gods, that your ally will force him to waste it. Elseway, hope for the best.

Now above level 6, it of course depends solely on what ultimate you can still, but generally speaking, try to utilize element of surprise on Sylas more than the flanking mechanic. You are squishy, so if enemy jungler will counter gank, you have a high risk of dying, so it'll be in your best intention to come fast, strike smooth and escape in time.

Also, don't be afraid to use E to jump over some walls (Topside or botside popcorn wall early).

Not much I can really say here. It all of course depends on the enemy ults. More tanky ults - feel free to be in the middle of fight. Assasin ult - utilize them to fullest. Supporting ults - well, now it'll be up to you to decide whether you'll use them on yourself or enemy. One way or another, always adjust to the situation - you are not static like other champions. Your play style is not bound to one direct path or preferred fight style. You have to be flexible, and make sure to see every chance you can to use another ultimate to surprise your enemy well.

All in all - Be flexible. That's all you need. Early game try to find the advantage, if you snowball then utilize it to the fullest without turning blind with greed. If you fall behind, turn tanky, don't pretend to be a god that will somehow turn the entire fight around. It's your own responsiblity to ensure that YOU will play as good as you can, and even if you can't, that you will at the very least avoid dragging your team down.

And the most important thing in my opinion - think for yourself. I'm not challenger. I won't provide you with god-guide that will solve all your issues. I make mistakes, and I also tend to feed from time to time. Try to adjust your build to the situation and enemy team!

With warm regards,
Wulfery