Disclaimer : I believe that most of the tips are related to a character, playstyle, so those will probably not work for any player. But if you somewhat share my profile (aka you trust in yourself and genuinely believe you are "better" than your current mmr), well, they should work.
I used to be 4.2k. That's what I consider myself being at, 4 to 4.2k. I don't believe I'm a 5k, but I don't believe I'm a 2k either. How did I fall all the way to 2.6k? Well, effing battlepass quests is mostly how.
Namely the infamous "mother nature's candy" (where you had to pick 3 arcane runes and win, of course, before they nerfed the quest to be any rune...). Back on the old map, this one was... insane. Anyways after 5~10 failures I resorted to spam Nature Prophet, ward the rune spots and just teleport there every 2 minutes. This went... very poorly.
I still managed to complete the quest with 3 stars after a while, lot of (legitimately) angry team-mates, lot of journey to low-priority, and a good 500~700 MMR loss. Yeah, a bit stubborn and a bit dumb on my part to do it on ranked, I'll admit.
But also the last quest for the WR set, where you had to do 5 (?) double hero shackle shots in one game AND land 100 hits with ultimate. This one didn't go well either.
Didn't go well at all. Probably another 700 to 800 MMR loss. But yeah, I still managed to complete the quest, in ranked, and got this beautiful set. Not everything was lost.
So yeah, I managed to fall all the way to 2.6k. And it has been very hard for me to climb back. Because I would still act as I would in 4k, namely do my things and trust my allies to... do reasonable things, I guess. And would occasionnaly get mad at them when they don't, when they build the most outrageous items, dive at the worst times, ask for wards no matter what, never ward anyway, and you know, just being overall... clueless. Which is a terrible reaction to have, it basically ensures that everyone hates everyone, no one wants to win anymore, the chat is spammed, and... afk in the pool ensues.
So, finally, here it comes, my top 5 tips on how to climb from 2.6 to 3.6k mmr (if you know you can do better than that) :
5°) - no rage, ever. Obvious one, but it is as potent as it is obvious. No. Rage. If you cannot say things in a very neutral, or genuinely funny tone, then stay silent. Flaming is basically throwing, even if you are carrying so hard and the only one doing so.
4°) - pick core position as much as you can. Safelane, mid. Yes, I see some streamers just picking random and steam-rolling every games, I'm not as talented as them. I cannot have impact when I'm position 5 (roamer 4, maybe). Plus, I'm nerd enough that I somewhat know the current meta builds, I do buy bkbs, I do save for buybacks, I do always have a TP on me, I somewhat chose my fights and occasionnaly even watch the fucking mini-map. Which is... mediocre at best, but still way better than the majority of what people do, when I let them have a core.
So I'm sorry if this tip sounds lame, but it has shown very clear results for me. Day and night.
3°) - buy wards. I mean just do it. If your support(s) are just bad with that, just get one yourself and put it where you know you need it right now. Like on the other side of the river for mid, behind the bushes for offlane, near opposing tower for safelane. It's 65 gold and it is worth it. Much more than spamming the chat with "we need wards" and angry X pings which will, most of the time, bring nothing but more tension / "my carry is bad" / "reported" etc.
2°) - try to lead. If you're the core, it's your items that should more or less give the tempo. Very simple things : push T1, back, roshan, push T2, back. Get T3, back. Microphone is better (faster), but typing does it too. Just stay neutral, and don't enter a chat discussion over things. Try to find the cooperative people and play with them, mute the toxic ones immediately (do not talk back, it's worst than useless : detrimental).
ie : on the first flame, whine, use of "well played" when we die -> mute
1°) - use the chat as a tool to win, do not treat it like a blog where you vent your frustration. "Get mid T2" "Let's smoke gank" "Get back" "Play safe", "Go farm this lane", "Let them have mid", those are useful and concise messages.
"You're fucking bad", "You have no farm", "Get fucking wards, dogshit", "My team sucks", "/All : Why does my team sucks so much all the time", "Your item is shit", "Stop fucking feeding", "Why did you die?", "No, seriously, explain to me why you just died?" (to which I sometimes answer with : ran out of hit points. Do not do that) those are useless messages, that will convey a negative vibe not only to the ones directly concerned, but to the team as a whole, everyone reads them and thinks "Fuck, here we go again". Avoid doing that.
Also, try to present things in a positive manner. Well played is to be used when someone actually played well, else it's a flame. And you don't want to flame, the enemy is the other team, not yours. You can totally win a game even with weird picks and people that are a bit lost as to what items they should make, if the morale is high and people are willing to cooperate. On the other hand, if everyone is salty as fuck and decides they'll just farm until your throne explodes, it's probably going to be a long and painful loss.
Probably a bit lame of a guide, especially on the "pick a core" part, since everyone cannot be a core. But again, it's basically how I did it, given my background. I believe the other tips are generic enough