Start playing few 1-3 champs spam them in ranked. By otp one or couple champs you are able to memorize their combos etc.. this way you dont need to even think how to perform combos etc.. which gives more room for decision making. You need tons of games under your belt tons of aggressive plays etc.. Through failure you will learn what item works better when, when you can one shot or stomp the fights, when to go for baron, push etc.. OTP mostly get high elo because they are going to be really good at the champ and most importantly having played soo much at their role they are going to know how yo approach, react etc.. to their lane or jungle. Less time you need to focus on your champ the more you will have for map awareness and decision making-figuring what to do would be better to end game faster. farming and use of skill should then become engraved soo hard u berly need to look or pay attention to it so you have time to look at the map(around the map whats going on etc.) and make decisions based on the information you get out of it.
