1. Find a consistent group to play with
One of the best ways to improve your elo score is to play with the same group on a regular basis. You can all find your strengths and weaknesses, learn how to communicate, and learn how to execute plays as a team.
2. Learn the champions
All of them. Know their names, their moves, their roles, and the strategies used to play them. If possible, play each champion when it becomes available just so that you know how they work. For champions you use regularly, pay attention to the dominant metagame for that champion, item build, and other updates.
3. Focus on getting better at CSing
Farm is incredibly important to your champion's competitiveness - and CSing is a huge part of your farming ability. Watch for creep waves and learn how to effectively last hit. If necessary, watch tutorials on YouTube to help you learn how to time your attacks.
4. Buy wards
Did you buy wards? Why not? If it's because you're a carry or a jungler - that's no excuse at your tier level. Good players ward and good teams ward, and in lower tiers, they do it no matter their role, because they know they might not be able to rely on their teammates to ward properly. And wards win games.
5. Internalize failure, externalize success
A big part of getting better at LoL is getting out of the "victim" mindset. If all of your losses are outside of your control and all of your victories happen because you're really just that freaking pro, then why is your elo still low? Are challenger tier players just lucky? No - in every game, decisions were made that led to the final outcome of loss or victory. When you win, look at what the other team did wrong and analyze why it led to their downfall. When you lose, look at what you could have done differently or signs you could have picked up on in-game to help you change strategies and turn things around. Changing your thought patterns and your internal narrative about why games go the way they do can help you improve.