He basically made the guide, check him out here with his youtube video:

https://www.twitch.tv/psychopathicporo

https://www.youtube.com/watch?v=5IgJ4PrNfOg

If you arent good at choosing forms, go rhaast every single game, even if you get shadow assassin first.
Sometimes, it is plain better going for shadow assassin, you can do that as well.

The Grandmaster Kayn OTP ollip goes shadow assassin most of the time, mainly because he thinks it is better in high elo right now. I believe if you arent confident on Kayn, you want to ignore shadow assassin mostly.

Contrary to popular belief, you dont need to fight a lot to get the form quickly. The system is complicated, but basically you get more orbs if you dont fight for some time. If you fight at intervalls like those:
Minute 3
Minute 7
Minute 10
you can get form at 10-12 minutes.
Also try to die for form if you can get it with the fight, it is always worth it.

If you go Conqueror Dorans Blade Ignite, you can kill almost everybody top lane lvl 1.
It makes the most sense to go this into the easiest matchups though, where you dont need to proxy, which are the ones listed in the easiest threat category. You just kill them constantly and get form minute 8 and its gg

This is what you will do in most matches.
First of, dying is not a problem, if you die for a good cause, like fixing the wave or getting gold or wasting the enemies time, it is good, just dont overdo it. Kayn wins most matchups when he has his items, the enemy items almost do not matter at all.

Proxying already at lvl 1 is the best, use Q to traverse walls and get between the tier 2 and inhibitor turret. There will be multiple scenarios happening here.

Scenario 1: Nobody shows up
Just proxy for free until someone eventually does show up or you dont have mana, you just go execute or fight them for stacks.

Scenario 2: The Toplaner shows up
Try to waste as much time as you can, they will lose far more even if they kill you. Get some orbs as well.

Scenario 3: The Jungler shows up
Again, waste as much time as possible, soaking pressure is good for your team.

Scenario 4: The Midlaner shows up
Funnily, most cannot kill you, so you can stack your passive and waste their time again.

Scenario 5: Multiple show up
The best scenario for your team, just chill and waste their time.

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If you cant proxy lvl 1,
try to slow push the wave, so they sit under turret, now you should be able to proxy. If they are freezing, just sacrifice the wave and proxy anyway, you would lose more not proxying.

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If you are in a matchup where proxying is bad, your plan is as follows:

You start maxing W, which does these things for you:
+ Long Range
+ Doesnt force a dash in
+ Poke is better
+ Easier to farm minions

In lane, you want to poke and farm with W, and get Form/Gold that way. Be careful of missing your W, it leaves you do be very vulnerable.

Proxying is still very good, even though you can now kill most champions.

Very Important:

Side Laning is the best thing you can do on Kayn:
Nobody can contest you if you hit your items, you push turrets and flank the enemies with E movespeed.

Once you get to Late Game, with you having 4 Items, not a single champion can ever contest you. You can kill carries with Flash Q R and should be able to win every game if you dont missplay, or they dont have a Poppy.