Help

Alright, so I ended Gold V this season. I didn't really play much ranked throughout the season and am trying to go for Diamond this upcoming season. I wanted your suggestions to see where I can improve my play. So far, I've decided to set up sticky notes on my desktop to record my mistakes per game, and I might watch replays as well. Also, regarding champions and roles, considering the new system for ranked, should I main a few champions for a few roles or what? Thanks.