It starts with accountability.
I get it, it's frustrating when your teammates aren't doing well. But believe it or not, a more productive answer to '[Champion] why are you feeding' or any similar sentiment is 'How can I help [lane, champion] do better?'
I will admit, I am not the best at playing this game. Many of us aren't. But a solid mindset that even professionals will tell you, is to make sure you realise that *you can only control yourself.*
Bot lane behind for x reasons? See if there's an opportunity to help them. If not, snowball another lane. Don't be an ass to them.
Jungle full of death? Ward safely, avoid chasing kills through there, and communicate your needs.
There is a reason that pings exist; to communicate effectively in a minimal amount of time.
I will admit, I am not immune to letting frustration out in-game. It took me a long time to learn that doing so is problematic, and only makes my life worse. So even if I'm tilted, I will do my best despite being angry at my team to figure out what *I* can do.
Dying a lot? Focus on playing back and staying safe.
Vision down? Place wards any safe place I can.
Yelling at your teammates, or even questioning their decisions after the fact, sets you up for failure. It's a team game, but for some reason (especially below Gold from my experience) there's this unfortunate mentality that you're the only one who matters. Granted, there are several design elements in the game that make this perception valid. But, since the players can't control Riot, we have to start with someone. And that's ourselves.