I would say its a combination of things.

There's outside of the game, where you read up to several guides and watch videos.

Then, in the game, I think spamming one champ over and over in your desired role until after every game you either start to feel like you're playing consistently or, where you are down to only a few mistakes per game is the best way. Make sure you know what went wrong each game and consider exactly how to stop it from happening the next game.

Once you figure out your problems (early/mid/late/decisionmaking) you read/watch more, and you ask others to make certain that you've found a solution.

Then, I'd say you continue moving further, move on from that champ, and remember what that champ's strengths and weaknesses are (since you've greatly improved at them, the only obstacles are counterpicks). Choose a new champ, and repeat. Sooner or later you will have a set of champions that you know by heart, and through learning them (your reading should help in other aspects) you have a better idea of counter picks, whether its your champ against theirs or vice-versa, and the role you are playing and the typical champs that play it and their strength and weaknesses overall.

Really, its just practice, and reflection, then key problem solving, and more practice.