You can't always win.
Think of it this way: if everyone on your team has equal skill, on average you'll each contribute to 20% of the game outcome.
There is a lot of variance though--one guy can completely lose a game with one bad play in late game, for instance. (But on average, with 5 equally skilled players, each contributes equally to the game outcome.)
If you are better than your current MMR, that means that on average, you should be contributing to more than 20% of the final outcome.
How much more depends on how much better than your current MMR you are.
Some games you will just lose. Lets say you are good enough to contribute to 80% of the final outcome on average. There is still a chance that your team fails hard and you lose.
That said, there are things you can do to increase the odds of winning. Focusing your team on objectives instead of running amok chasing kills, for instance.
Play in positions that let you hit many areas of the map to help teammates who are having a bad game get rolling (mid & jung).
If you win your lane hard, your team will be more likely to listen to you. So the basic steps are:
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Figure out how to win your lane most of the time.
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Figure out what your team as a whole needs to do after the laning phase to win the game.
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Perform really well early game so your team is more likely to listen to you.
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Lead your team; tell them where to group up and what objectives to take. Tell them when to fight and when not to fight.
- They won't always listen. Try to find the right balance between firmly suggesting the right course of action, and not being a dick. (If you say something like "HOLY **** YOU SCRBUS! Get mid NOW idiots!" they probably won't listen. If you say "Ok guys; time to group up mid & take some turrets. Stay behind Leona and don't go in until she does.", and maybe "Leona: We aren't strong enough to turret dive, only initiate if we won't be under a turret"). Be careful of being too much of a dictator though--even if you are right, people often don't want to be told what to do. Try to guide, not force.
- Sometimes they will listen to you, but not execute properly. In the above example, Leona may get excited and engage under a turret anyway (even though she meant to follow your instruction, she tunneled and went in). Or your teammate interprets "stay behind Leona" as a general suggestion, but wants to poke the enemy so gets in front of her every now and again; enemy engages on him, blows him up, and now you are all either running or in a 4v5.
There isn't much you can do about this. If it happens, make a judgement call as to whether it would be helpful to point out what just happened, and what to do better next time. ("Ok, that went poorly, but lets try it again; this time make sure to always be behind Leona please.")
Be careful of pissing people off though--they will be frustrated if the plan isn't working.
Try not to do anything too complicated. Generally things like split pushing work out poorly in low-ELO soloQ. The players won't have the mechanics to execute properly--keeping pressure so the enemy team can't go get the split pusher while not getting engaged upon is hard. And most of the time they won't even remember to back ping the split pusher when they stop pressuring (and the split pusher won't remember to keep an eye on the skirmishing to see when the enemy comes for them). Etc etc. That was just one example--any sort of complicated strategy (more than 1 or 2 things that need to work together) will more often than not end in chaos.
So, stick to simple things: "Stay behind the tank, go in when the tank goes in."
"Every time the wave pushes up to their tower, ranged people hit it a few times, then we back off. Go all in if they engage on us."
"Every time the wave pushes up to their tower, ranged people hit it a few times, then we back off. Run if they engage on us."
And so on.
