Just farm CS and engage as much as you can without taking more damage than your match-up. Get Darkin ASAP.

Always start Raptors.
Get a quick leash from your Midlaner(remind them not to steal any of the small ones, if they do, don't give them ganks. Nah jk. If they do, be toxic af and go bat**** crazy but still go for ganks especially when their match-up has low HP.)

Get Lethality Marks on Shadow Assassin, and AD Marks on Darkin. Lethality works best on SA since you want to decimate your enemy as fast as possible, and AD runes work best on Darkin since you have the FoB Keystone lol.

Always get Thunderlord's on Shadow Assassin since you need to burst your target as fast as you can, and FoB/Warlord's Bloodlust on Darkin depending on your playstyle(Get FoB if you're the type to engage as many times as possible to get a quick boost on your Transformation, and Warlord's if you just want to farm up on CS without having to recall every few minutes)

Shadow Assassin doesn't really sit well with prolonged Teamfights, so always go behind the enemy team and rape their squishy carries

Darkin is more of a bruiser(Kinda like Darius), so be sure to ALWAYS land your W on multiple enemies since it will act as some form of signal for your team to engage the enemy team

Pros: Kayn is extremely OP once he transforms into either forms, and can decimate the squishies if he takes SA, and can start teamfights and WIN them if he plays it well as a Darkin

Cons: Early Game is ****. Your spells don't do much damage to your enemies unless you have Warrior Enchantment(wtf? How'd you get Warrior on level 3 lol) And if you get zoned out(early, mid, late) by the enemy support and CC-locker, you're screwed. There is literally no way to get out of being screwed if you get focused by their heavy CC.(Darkin AND Shadow Assassin)