I don't honestly know if my method is the best. But I stuck to 2 - 3 champions, each only played in a single role (despite being able to go into another) and became very good using them despite the team composition. That may sound horrible, but I've seen time and time again that even if a team comp is bad, as long as it's not contradictory (like if you have Ezreal and Jayce as bot and mid, but then Skarner and Diana as top and jungle...) being really good a a few champions far exceeds playing a champ for their utility.
I won many games with Soraka support against every possible matchup, and I got first blood a lot, and it was through my aggressive play with her. I went Karma Mid for about 15 games, and won about 13, that included going against leblanc, zed, xerath, twisted fate, orianna, akali, and talon.
So choose 1 handful of champs you LOVE to use, and use them, all champions are in a good enough state to get into platinum, 100% true. Next, if you are anything but support, look at your CS at the end of every game. After about 5 games, create specific #s you want after every 5 minutes in the game. Idk how, but that somehow made me win more, when I could have sworn that I was always on top of my CS beforehand. Thinking strictly about CS, and NOT poking or doing something risky for a kill, ends up better. You want to get fed, but you don't also want to feed.
ALSO, personal memo. I tend to keep in my mind that every single death, is another reason why you should lose the game. Think of it that way. Every time you die, you messed up. If you tried to save someone, and you both die, it's your fault. (Why were you trying to save them? Were they in that position because of you?...if not, then again why risk death for something that wasn't your mistake? And if it was because you weren't around to help, stop getting sidetracked and realize that maybe they pushed lane because they saw you behind them and thought they had the support).
Idk my mind goes on tangents, but focus on 3 - 5 champions, count your CS like an arcade high score table, and mute people. talking to people has almost never helped me in a game where a ping wouldn't have been just as good.