I really didn't expect writing a guide will take this long, so I'll just leave you guys with a short version for now and I will work on this next. What will be here in the next few days: Item explination, How to Choose Form, All the Clears.

Clear options:
Full Clear: Red ---> Krugs ---> Raptors ---> Wolves (use smite here) ---> Blue and Gromp (Ideally you would do these together but even I struggle with it so it's fine if you do them separately) ---> Scuttle/Gank. You want to save your second smite for the Scuttle, but with the nerfs for it in Season 12 it doesn't matter if you use it on gromp or not.

Items:

Shadow Assassin: Prowlers gives a lot of kill potential, but if you struggle with active items go for Duskblade, they are pretty much the same item. Goredrinker is good if you think there will be a lot of teamfights and you take it if you go Conqueror Blue Kayn. A lot of players like Eclipse for the shield + Armor Pen passive, but in most cases you will not need more Armor Pen from it. Stridebreaker is the worst out of these since you already have slow on your W and Chilling Smite, so I personally recommend Goredrinker if you want an Ironspike Whip item.

Rhaast: Always go Goredrinker, but if you want to be edgy like the champ you are playing... Eclipse is good on the famous "Rhaassassin" build, tank Mythics for a Tank Rhaast build which is just no. Divine is not bad on Rhaast but you don't really auto attack, you more so focus on spamming your abilities. Stride for the slow, but with W knock up, Q, R and Blue Smite, your stick potential is already insane.

Choosing form: Shadow Assassin/Blue Kayn if they have 3+ ranged and the 2 close range will still be kiteable/killable by you or your other teammates. Rhaast 3+ close range or there is a big threat on the enemy team you can kill as Blue Kayn. For your first few games I recommend going "Purple Kayn". Rhaast can also be played in every situation, although suffers the same fate as other bruisers/tanks aka True Damage or Vayne. Still if you are new to Kayn going Rhaast every game is better because he provides more for the team and can come back more easily than Blue. Unless you are smurfing or are a god at Kayn don't blindly go Blue every game because you will int and your team will flame you.

I'll work on this more tomorrow and other days, and if you want some more guides on some champions I'll gladly make more. Video on Kayn coming out soon too.