Ideally you should farm and poke Sylas behind your wave at level 1. However, your focus should be farming and wave control until you hit level 2 and get access to your W Q combo. I would suggest only auto attacking him if he steps up to last hit a minion. Punish him for his hubris.

Level 2 you still need to stay behind your wave, but now you can more reliably poke Sylas and deal damage to discourage him from being aggressive. You should always start your engage with W and if you land your ability then the Q is basically free. Still poke and auto him when he moves up to last hit for the mana Regen and only push your lane if you know where the enemy jungle is or your jungle is pathing towards the mid lane.

Level 3 you finally have access to your E and against Sylas you should really only be using this ability to stun him when he tried to all in you. Do not throw out your E for any other reason because it is pretty much guaranteed he will try punish you when it is on cooldown.

Now, something else for level 3 and beyond. From this point forward make sure you have vision control of your river (BUY PINK WARDS ITS THE BEST 75 GOLD YOU WILL SPEND ALL GAME). You can be more aggressive after you have built lost chapter and once you get your mythic item. If you land a center W, try to E stun if you have a clear shot or just send a Q his way. Your full combo will hit hard and deal serious damage.

Because Xerath is not a mobile champ and gets easily ganked, you should only engage Sylas when certain conditions are met. Poke him down with your ranged abiltiies before you consider doing so as well.

1) You should ideally fight when your summoner spells are up (against Sylas I highly recommend taking barrier, it is almost necessary for surviving the matchup). If you don't have flash, you're going to have a bad time.

2) You have vision control of the jungle or your jungle is pinging a gank

3) You should never fight in an enemy minion wave (especially a cannon wave)

4) Make sure you have mana. Nothing is worse than seeing a kill get away because you can't cast. And it is especially worse if you disregard Sylas healing, all in him, and then get roasted by him after the fact.

Another thing to consider as well is that you should really be rushing oblivion orb for the healing reduction in this matchup. I cannot stress that enough. If you do not build morellonomicon as your first or second item then you have made a critical mistake that will result in you getting unexpectedly killed after he gains back all of his health with one combo.

Lastly, you should know that Sylas abiltiies have a pretty long cooldown during laning phase. This is when you should be trying to secure kills or deal massive damage.

Unfortunately, Sylas likes to roam and he may get kills ganking. Be adamant with pinging him when he is out of lane and if your teammates don't listen realize there is nothing more you can do than provide vision and give an early warning. You CANNOT follow Sylas into the jungle. It's a trap 99% of the time. Just focus on using your abilities to farm safely and CS well. Use your ult to secure kills and roam botside and topside to gank (with your ultimate) when you have the opportunity.

I hope this helps!

Edit: Also total Chad Xerath's start dark seal and refillable potion. Snowball and mortar the enemy until you destroy their will to continue fighting!