The best way to get good is to get aware
As somebody whos not a smurf it seems that the biggest problem with plats/low diamonds is that they are completely clueless with 0 map awareness.
For yourself in your own lane, pay attention to the enemy jungler pathing and where the enemy jungler is. Know when its likely to get ganked and play aggressively if you know the enemy jungler is on the other side of the map. Do not ask for ganks if you did not put down vision and you are behind, all that will happen is a losing 2 v 2 counter gank and a free double kill. Junglers should never gank a losing lane with no vision and no idea where enemy jungler is.
For your teammates, b ping them as soon as you know they are getting ganked/roamed on. if you suspect your opponent is roaming, immediately ping. if they don't want to listen, they can stay trash and its not your problem.