After buying a Sona mousepad I've gained almost 1000 lp so that seems like the easiest way to improve.

eh who knows but my guess is watch your replays, write down your mistakes, look for common problems to fix, address them with new habits to build in next games

as an example some notes from a year ago: legendsofleague.gg/league/post/22/S12%2520ending%2520notes/

for choosing which games to watch, close games are best, especially losses and games where you lost bot

for finding mistakes deaths are good to look at, especially your first death, as well as lost fights while the gold graph is still pretty close; there could also be times when you could kill someone but don't, but those aren't as obvious

a good way to phrase a mistake is "they did this, so I should've done that", eg they hit 2 first so you have to back off, not that you have to say it exactly like that but you want to see their threat and what you should do about it, like ok they can gank at 3 minutes so I need to ward before pushing

then eventually you notice consistent problems/things you need to do, and think about what habits you need to improve in these areas - this is where having *x happening in game, so I should've done y* is helpful, because you think about it while reviewing and reflecting on your games, so then in your next game you see the sitaution you already analyzed and automatically know what you planned to do

finally for sona in particular the champ pretty much plays itself, I'd focus more on what enemy champions are doing and not allowing them to make their plays, a good rule for sona is don't get cced, especially not in front of their adc, and really in your ideal trade you don't get hit at all, you just q them for free