As someone who mains ADC and got to fairly high ELO on it, I adopted one way of thinking that really helped: farm is the end goal for every action/choice that you make. Should I harass? Should I engage? Should I push? Every choice should be made with the intention of giving yourself a better position to farm in. Obviously this is true in every lane, but as an ADC much more than mid or top.
Although its difficult to reach this point, but obviously mechanics is number 1. The reasons are obvious, but one more thing comes out from having excellent mechanics: you don't have to concentrate as hard to CS when you are doing it naturally and well. It has happened to everyone where they are so concentrated on getting good CS that they completely miss the minimap, or don't see a fight happen two feet away from them. Once your CS gets to a level where you can do it without spending as much brain power on it, it opens you up to do other things better e.g. map awareness, positioning, and ability to capitalize on mistakes of your counter-parts

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