Step 1: Change your mentality.
The number 1 thing that I think about when it comes to this game that calms me down is "What is your end goal of playing this game?". and my answer? Is to have fun, and if I get better as a side effect, then so be it. I do not aspire to be pro, nor will I ever be pro, so that guy in top lane that is 0/8 vs 600 stack nasus with his tower gone that still runs in 1v5? lol IDGAF about him... he is the team mascot. He wants to win. He wants to win soooooo bad, and thinks he can turn it around, but just lacks the ability.
Step 2: Learn to deal with other negative players.
There are three kinds of people you're gonna have to deal with that you will want to rage at:
Type 1: PPL who think they're good
"It's the jungle's fault I lost lane!"
"I got ganked 5 times and my jungler didn't come once!"
"We woulda won if you just went in with me!"
Best way to deal: Don't directly insult them, even if trying to give constructive criticism. They can't deal with criticism of any kind.
Type 2: PPL who don't give a damn
"Whatever"
"idc"
"AFK"
Best way to deal: Don't directly insult them either. They're going to AFK, then you're going to lose for sure.
Type 3: The ragers
"You're bad and you should feel bad"
"Why did you go in 1v5 when you saw we weren't nearby?"
"What the %*$@ did you do that for?"
"Why were you there?"
"Where were you!?!?"
"Uninstall now plz"
Best way to deal: Mute and report.
Or if your zen is particularly strong that day, try one of my other favorite options:
"I heard that if you yell at your teamates it makes them play better."
"Oh, you mad bruh? :D"
"How mad are you?"
"Whatcha gonna do about it?"
"You gonna cry? :D"
"You gonna whine and cry that I lost the game for us? :D"
"How about after game we hug it out like bros?"
"OMG your yelling is making me play worse!"
"Now I'm playing bad because you're yelling at me!"
"Can't play good cuz I'm being yelled at :("
"Your yelling is negatively affecting my pro LOL skills"
"Don't worry bruh! :D I know i'm 0/8/0, but I will beat him in the next 1v1! Don't worry!"