I'm not a pro or a coach. My claim doesn't require me to be one, I'm just a regular diamond 1 player. (Petyr#6969 if interested)

Having a mic is almost necessary, but having someone on your team who talks in a way that no matter what happens (even if you're 8-1 down) nobody dares to spoil the mood and blame anyone else. You don't have to be aggressive about it, but all you need to do is have a positive vibe to the game. It's gonna help in having fun, keeping the morale up which ultimately increases your chances of winning. Worst case, you and nobody else gets toxic, nobody gets tilted and is calm enough to pick up and learn a thing or two from the game. Even when you're 8-1 down, the positive attitude of almost everyone will make the team work together and get at least 6-7 rounds even if we lose. Best case scenario, you build up momentum from one of those rounds and there's your comeback.

How do you do it? Talk nicely, say hello to a couple of players in agent select. Be more agreeable, and take blame and responsibility "as a team." If someone makes a mistake, instead of saying "Reyna shouldn't have dry peeked B-Main, that was stupid" you can just say "We shouldn't have committed to B so early. If someone still tries to point the finger, it is alright to agree with them, like with a "Hmm" so they can be calm that it has been addressed. But immediately, move on and talk about the next round (probably the current round since everyone's blaming in the buy phase/first 10-20 seconds) and give them a strat, or ask for it from the same person who was blaming. I'm not sure but this might sound too difficult as I might not be very good at explaining it. But it's all about keeping the positive vibes and yet, not having the "it's okay if we lose" mentality. The goal of the team should be to have a bit of hope (if you're losing) and build momentum from one of the rounds you win. That's what you say when someone votes for forfeit or starts to throw. I'm not the best people pleaser so if this post gets any attention, some of you could possibly add on to what helps keep your team together.

The best test or a measure of success that everyone's on the same page and the right environment is set is when nobody looks to blame the bottom fragger and everyone acknowledges every player is helping.

The added benefit and the reason why I said having a mic is "almost" necessary. Even if a couple of players aren't using comms all the time. They are gonna focus just as hard on the game and most people play better when they're not tilted, specially when they are not expressing themselves. I've seen this in games that players who aren't using their mics all the time perform so much better when nobody's fighting. Else, most lose morale or are too worried that they're gonna be the next one they don't make plays that are a bit on the riskier side which are more rewarding. For higher elo games (I've played in immortal lobbies) you absolutely need 5 players to comm properly.

Now I might be taking it too far but I also think that our mind suppresses bad memories and when your morale is low or you are playing when you're pissed whatever happens in that game is forgotten. You might not learn anything, and it's the small bits and pieces (could be duels at specific angles, or reacting to util) that help build your muscle memory and even in full consciousness affect your game.

TLDR: Be nice, positive vibes on a server go a long way.