- vision:
are you buying enough wards (yes, pinks too) and placing them in smart places? if you don't already, try looking for opportunities to ward the enemy jungle, not just place a few in your lane. work that trinket/sighstone! also remember to get red/blue trinket asap laning ends
- csing:
don't be bad at csing like newt. kill chickens or golems or whatever every time you can and just practice last hitting. you can try that 10 minute challenge thing against intermediate bot mf if you want, it helps. if you're support you could try and learn how to set up cs for your adc
- don't die:
you don't need to chase after every kill, sometimes sending someone to base is enough to put you in the lead, just push the wave, shop and you're already winning. use wards not to get ganked, don't try to 1v3 outplays, don't be out of position for no reason, etc. basically, start thinking that you should be dying way less and start playing around that
- learn to get carried:
if you're like 0-2 in lane because you're like against teemo or something, learn not to be a bigger problem for your team. your jungler might or might not help you, you can't really do much about that, so stop making it worse. if you got beaten two times, it's not likely that you'll defeat them on the third try. just play far back as to not get harassed, and farm under turret as it's slowly chipped down. it'll go down eventually but it's better than you being killed and it going down anyways. if you can't farm at least get exp. if they freeze the lane you're screwed but you can try and roam somewhere and maybe get a turret, stuff like that.
- objectives:
always think of objectives. you got a kill? can you take mid? can you take dragon? are enemy adc and mid bot and you can rush baron? sometimes it's only a jungle camp or like scuttler or maybe a big wave, but that's fine too
- use pings:
ppl aren't mindreaders, be that person that uses pings a lot. just don't spam BACK BACK BACK when jungler is doing you a favor and pushing your wave after a kill
- play easy champions that scale well:
forget about zed, yasuo, azir, kalista, whatever. these champions not only require you to have good mechanics but also really good decisionmaking which is something that if you're in plat or less you probably don't have lol. stick to the easier and less glamorous champions. you can look in lolking or champion.gg for the ones that have pretty good winrates. so you know, malphite, jinx, janna, that kind of thing. there's gotta be a champion like that that you find fun.
- develop a small but reliable champion pool:
don't play like a billion different champions/lanes, just pick a lane and learn to support and that's it.
- don't talk if you're gonna say something negative:
don't say "why", "gg" or any of that shit. morale is silly important in the game and i've seen a lot of easy games being lost because someone gave up and ragequit. just don't say anything unless it's something like "we can drag", "oom", or whatever. don't reply to ragers and also don't be a kiss ass lol