TL;DR:
-Don't bring up the "skill" of your teammates ("you're not very good", "you're really bad"). Me being Diamond in Solo Queue, I like to take a break from Ranked, and I'll goof off on a champion I don't play often, and in a role that I don't main. I never do poorly, but when I'm not playing my best, and I do something that a toxic player doesn't approve of, they'll often say "how much did you buy your account for" or "wow Diamond is that easy to get to, how the fuck am I not there yet cuz you're really bad". That's a guaranteed report right there. Instead, say things that are neutral, like "I need you to help me", "I'm counting on you to do this", "We need you to start doing this so we can get ahead of the enemy", and actually seek to cooperate with teammates instead of insulting them to make yourself feel vindicated.
-Don't comment about bad things when a different course of action wouldn't have changed it. Instead, suggest meaningful plans/actions.
-Address what is happening in game, not the player. Say things like "can you back up so we can team fight together" instead of "stop overextending like a bronzie" followed by spamming '?' pings on your teammate.
-Talk/discuss with your teammates without dismissing what they are saying. If they are bringing up a point, saying "lol", "lmao", "you're so stupid", is the easiest way to earn a report, because it's not addressing what they said, it's just being a complete asshole without backing up what you think. Actually talk out the points in chat, and make it meaningful. Some players do that, but also toss in some negativity. Avoid saying "we would have won if you weren't dicking around split pushing all game"; instead, say "we will win team fights, please group up". Avoid saying "you are completely useless"; instead, say "we need you to start doing this, because our team comp is better when that happens".
-The most common reports I put out are when players say things that are ad-hominem (attacking the person's character in order to discredit them), use superlatives to insult ("you are NEVER grouped", "you are ALWAYS dead", etc.), and don't own up to their mistakes ("why did you overextend?" "lmao shut the fuck up we aren't going to win anyway so why does it matter")
Some contextual situations for when I put out reports:
I play a lot of Treeline, and I duo with a friend. Game is going well, I am playing Master Yi in the jungle, my friend is playing Ashe, and our random is playing Singed. We're all doing well, mid-game we are all about 4/1 or 4/2, we have tower advantage, my friend and I have killed Vilemaw twice. The Singed dies in a 1v3 and says "you guys suck". Wtf? We ask "how" and "why do you say that". He starts complaining that "Ashe is only pushing towers" and "Yi only comes in to clean up kills after I am dead". While my friend and I are explaining to him "that is exactly how our team comp works, and we are winning the game by doing that", he sprinkles "you two are bad", "stop waiting until I'm dead to do something", "suck each other's dicks harder" in chat. He kept raging about this, even though we were winning the game by a landslide. The enemy team only got 1 tower over us, and I think the end scores between the teams was 15+ to 4, our advantage. We reported Singed for verbal abuse because he was berating us when he was making poor choices in positioning and we were winning the game hard.
Had another game where I was duo'ing with my friend again, I was playing Nautilus in the jungle, and our random was an Ekko. There were times when the enemy team would 3-man-invade my jungle, my friend would come to respond, and we'd both make it out alive (nobody on either side died). Our Ekko is completely oblivious to this, so he is still out in lane. The enemy team comes out of the jungle and catches him, he dies, he sees my friend and I in spawn, and says "wow you two suck, how come you aren't helping". We explain to him what happened, and he says "lol, such bronzies". Later in the game, enemy team would again, invade my jungle, force my friend and I back to heal. The enemy team would then 3-man siege a tower. Our Ekko stays under the tower, spam pings it 7 times, gets tower dove and dies, then says "wtf noobs DEFEND THE TOWER". I explain to him that "we did not force you to stay under the tower, you could have left" and he retaliates with "you two are such trolls I am done with this game". He then says in all chat "Report my Nautilus such a troll". My score at that time was 4/0/10~, Ekko was 5/4/6~. He says in team-chat "don't worry I will carry this game". We ended up reporting him for verbally abusing us in chat for his own mistakes.
Another game, I'm playing solo. I am Tristana in a solo lane, my teammates are Ekko in a solo lane, and Jarvan in the jungle. The enemy team is Hecarim (jungle), Lissandra, and Teemo (my lane opponent). I have a decent early game against Teemo, but at a certain point, Teemo gets a lot of AP and is able to reduce my hp to 30% with just his Blinding Dart and two auto attacks. He also has Ignite. Needless to say, I can only farm for a bit and then recall to heal up so that I don't feed Teemo gold. So, the times that I go back to spawn to heal, Ekko and Jarvan get in fights with the enemy Lissandra and Hecarim, and lose the 2v2s. They look for where I am on the map, and see that I'm in spawn healing up. Both of them start saying things like "Tristana you are completely useless", "Trist why are you in spawn at full HP", "Trist, why are you shopping when a fight is happening". I have no problem with the last 2 phrases. Problem was, they kept spamming "Missing" pings on me in spawn whenever I went back. When I explained to them "I was too low to help out, I'd just die", Jarvan would say "no you are just lying you piece of shit" and Ekko would say "no Trist you are wrong, stop talking". Then full-on 3v3 team fights would happen....
...as a squishy ADC, I can't get close to any of the enemy team at the start of the fight (Hecarim, who has Trinity Force and a terrify ultimate; Lissandra, who can focus 2.5 second stun/snare combo onto me; Teemo, who can blind my auto attacks for 2 seconds). That being said, I have to skirt the edge of the fight to make sure I won't be focused, so that I can consistently put out good damage. Jarvan would go out and do his job, but I'm not able to finish the fight because Hecarim would save his ultimate for me. Lissandra and Teemo would kill my Ekko. When the fight is over, Jarvan would say "Trist you're not helping at all" and Ekko would say "why is Trist never in team fights". Jarvan starts arguing with me about using my Buster Shot to keep Hecarim off of me, even though Hecarim is CC-immune during his ult dash. Jarvan then starts saying "you're reported", Ekko says "stfu Trist, you're wrong, stop". They continue to spam ping "Missing" pings on me when I'm in spawn healing, they argue with me in chat about how much HP I have when I first enter spawn, they call me a troll for going back to spawn to heal and shop, Jarvan says "We will see the end-game stats. If you haven't dealt enough damage, you are reported". It's abundantly clear what I reported both of them for.
