This is something that you should be doing before the game loads, watch carefully the enemy comp, and your team.
Keep this in mind: hitting melee champions you get red orbs if you hit ranged champions you get blue orbs.
If you are new at Kayn I recommend you to see how many melee and ranged champs they have in their comp, if they have more melee champs you get Red form (Rhaast) if they have more ranged champs you get Blue form (Kayn), and change runes according to the form you are gonna use that game.


When you get more comfortable with it, you can choose your form based on the question "how many of them can I one shot or kill in a few seconds?" (that's usually what I do to choose it xD) if the answer is 3 or more you can go blue and win easily, for example, 4 champs that give red orb but are squishy, you still go Blue and viceversa.

WARNING CAMPS AGGRO RADIUS IS DECREASED SO CLEAR IS ALMOST THE SAME BUT CHANGE THIS: AFTER RED WALK TO KRUGS INSTEAD OF E - NO DOUBLE CAMP IN BLUE SIDE.
Kayn is one of the few champs that can full clear before Scuttle Crab while still having smite up, you can clear before 3:10 if you practice a lot and it will help you so much in your games!
This is a full clear by Karasmai, probably the best Kayn player I know and the one that helped me make this guide with his videos and explanations he gives on Twitch and YT. Play Video
After you clear W on Scuttle and before it hits use smite to destroy the shield.
Try to remember the number of hits to give each camp and the movements he does, if you don't get a perfect clear time in the beginning don't worry! Just practice and you'll get there really fast and climb elos!