How to improve on decision making in League of Legends?

[Chilies]

I was wondering if anyone could give me advice in LoL. I started playing a long time ago but recently I've been getting more serious on improving. I was wondering how others improve on their decision making and just core aspects of gameplay. For me, I use use to just watch streamers and more or less copy how they play, and that has gotten me pretty far (Diamond 1 as a 1 trick pony), but now I want to expand my champ pool and play other roles but I've been struggling so I'm looking to see what others do.