Okay I'll give some more specific tips than these dudes:
In season 7 I played ranked all season and there are 2 things that mainly got me from bronze 5 to Silver 5.
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Play roaming champs. TF is a great start if you can use his ultimate effectively. I personally played mainly Talon mid/Kled top and that worked pretty well for me. Being able to get a lead in lane (by abusing those champs' early power) and snowballing it into other lanes with roams gets the whole team ahead and makes it super easy to carry a lot of games.
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Okay, imagine a picture of a person shooting a deer (we'll just say for hunting purposes) while the bullet is still in the air. Now imagine over the guy it says "you". Imagine over the bullet it says "morale". And over the deer it says "my teammates". You need to literally shoot morale into your teammates' assholes to get them in a good mindset to win. Here are some lines that I love:
"Just stall for late game, we'll win fights!"
"You'll be useful later on, just farm and get strong!"
"You're doing well on [insert anything they're mildly doing well on], keep it up!"
"We're winning on [generally kills/towers], we can win off that lead!"
You can say any of these even if they're not necessarily true, and it will usually still work. I've saved myself from multiple feeders just from saying these kinds of things.
Also the occasional "?" in all chat to tilt the enemies is always a plus if need be ;)
Good luck brotha!
PS: Don't look at this account, it's my second one. My first one got memed. You'll just have to take my word for it that I got to silver.