When youre playing Kayn, you first have to understand your goal. Look at their team and decide which form would be best for dealing with what they have. If they have tanks and CC, dont bother going SA. If they are all Squishy targets, thats a good time to go SA. Once you know your goal you can start to map out what you want to do.

Look at their laners and Jungler and whether they are Ranged or Melee champs. For my example assume you are going SA. I would look at Bot and Mid. If they have a Mage mid, and a Marksman bot, those are the people you want to camp early game. You need your transformation as soon as possible and if you spend all your time top bullying a Darius, then you will have to wait 3 extra minutes to get the transformation you want. However, dont completely ignore Melee champs because you want SA. You can still help teamates fight them, as long as you spend more time on those ranged champs.

You want to start Raptors, without a leash. You can easily solo raptors if you start Q. Then take E second and use it to heal and take red buff. E will be up again on your way into your Blue Jungle. If the enemy jungler is someone who is weak early, look to invade at level 3. Just dont do so earlier because having your QWE is essential for safe invading.

Pros
-Good Damage
-Easy to get the hang of
-Healthy Clear
-Creative Ganking Routes
-High Mobility

Cons
-Useless if he falls behind while SA
-Easy to counter
-Most Damage comes from Skill Shots

Here is a small clip I put together of the few Kayn plays I still have on my computer. I chose these ones because it shows how you can use basic Kayn to get early kills, what shadow Assassin Kayn is capable of, and what you can use Rhaast for to protect your teammates.
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