Just watch your own matches until you can consistently spot at least 10 different mistakes each time you watch your own replays. Use this site to get said replays : replay.gg
You can also try to 1v5 random bots in Custom-Matches, setting them to Intermediate-levels, then solve the puzzle of how to win, especially if you hit the random button & picked a champion that cannot 1v5 very easily on their own (such as with most ADCs against any CC-Heavy composition). 'Tis do-able by the way, just requires strategy, and some cheese, and this item...
(and also this one :
followed by a
)
The champions that can easily 1v5 may not be sufficient challenge to win in a 1v5 against Intermediate-Level bots (like it's easy to do as Swain for example), but if you cycle around champions, you can also pick up on hidden strengths/weaknesses (for example, I went to Yasuo in a 1v5 against bots, thinking that Yasuo was broken, only to get shat all over by bruisers, thus learning that bruisers like Garen & Jarvan & being gang-banged whilst a Support-Léona has him chain-CC'd shits all over him).
Also, the match-making is a luck-system, and the random-generator seems to be somewhat time-based, therefore, you will get better matches at certain times of the week or day than others, and another mistake that people make is thinking that consistently winning their lane should get them higher elo. Well, who cares if you crush your lane, because if some othe lane on the other team crushes their lane even HARDER than you crushed your own lane, well, your mistake was not roaming to stop that crushing, getting your team-mates fed so that you could worry less about them being able to handle themselves without your presence, and if you can get your own team-mates fed hard enough, you can get them snow-balled to the point where you can go back & concentrate on your own lane without worrying too much about the rest of your team feeding the shit out of your opponents followed by four-man surrender-voting you into Bronze (god I hated that and it happened to me TWICE this season).