What mechanics do you think your weak with?
Find a champion who requires you to use that mechanic.
Go to normal play that champion a lot until your winning consistently/having a good impact on matches.
Vision is a really important part of the game.
People make most their "Tier Lists" with the assumption that who ever playing it is pretty high skill. Often times its better to play things that are more reliable when your new/lower skill because that lowers the # of feast or famine games where your big impact or 0. winning 1 sidedly because you did well...but losing 4 games cuz your just doing meh is gonna sink you not help ya climb.
A good amount of this game is knowalge. Knowing items, champions. Personally I used Free week and played every champion whos kit I did not know a basic understanding of Passive qwer. If you can accomplish this by reading then hey more power to you just use the wiki and save hours and hours of time,
I needed to get a feel for it to the point I can play off muscle memory. I also had to get used to working as a teammate rather then treating the game like a 1v5.
End of the day the way I see it there are 2 main ways to improve your rank. Up the frequency of your good games and increase your micro. And to Lower the frequency of your bad games and up your Macro.
Good micro will win you lanes. good macro will keep you relevant if you lose lane or get dicked by camp or what ever.
Doing the right play in theory... despite it not being realistic for a match is such a common mistake. I did X which was the right play but because of Y it was not. Typically Y is a teammate.