Sylas has an extremely bad early clear. If you decide to go Sylas jungle, make sure to have your team mates stand at the river entrances to avoid dying to invades. As for pathing, you should start red buff when possible to raptors -> wolves -> blue -> gromp/scuttle. You can start blue buff as well -> gromp -> raptors -> red -> scuttle/gank.

Play around your powerspikes. Sylas is really weak early so try to take fights only when you know you can win them. You generally powerspike at your jungle item (runic echoes), lichbane, etc. Basically anytime you complete an item, you kinda "powerspike". Powerspike just means you suddenly become much stronger due to the extra effects granted by completing specific items.

Sylas also powerspikes at lv6, lv9, lv11 and 1v16. At these levels, Sylas gains a load of extra stats on his skills, making him super bursty, like an AP assassin.

After the removal of animation cancels on his aa and his e2 q combo, there isn't really any special moves left. You just use your abilities at the right times while keeping in mind your passive can charge up to 3 times, so to maximise dmg, it's best to use a stack or two when your passive is fully charged, so that you can keep charging it. The best combo I would personally recommend is e1 e2 aa w aa q aa aa.

E1 to gap close and e2 for the cc. w to gap close again and q for the slow.