Some steps that you can take to being less toxic
I am going to write this post to try and help out those toxic people who want to improve, but have no idea how. Of course this won't be all the possible things that you can do, but I'm just an average person so I'm not going to get all of them. The steps will (hopefully) be in order of difficulty to pull off.
Step 1: Just try really hard to be positive. It sounds dumb but if you sit down and picture how bad getting a permanent ban on your account is, it could motivate you into being positive for at least a few games. Starting out the game being positive an nice can help keep you positive and nice over the course of the game.
Step 2: Mute anyone who even starts to be toxic. "But it's reducing my communication with them". Good, that's the point. If some a$$hole is flaming everyone for every mistake, are you really going to miss out on communicating with him? I know that personally this works really well for me. If someone starts throwing around some sarcastic comments or starts to insult some people, I just mute them. Toxicity for me is infectious, so I reduce the amount around me.
Step 3: Play with friends. Playing with friends could help you be less toxic, but I personally do not recommend this. It could surround you with fun people you know or it could put more trolls than you want in your game.
Step 4: Rebind your chat key. Have someone else rebind your chat key into a key that you do not know and you never use. That way you won't have any idea how to type in chat and be toxic to someone.
Step 5: Check and see if your champion pick is the issue.
If you are picking a champion that absolutely requires a first blood to snowball the game and win, you might have some toxicity problems. The pressure of having to snowball and win lane can make you be toxic and/or make you be super toxic if you lose lane and begin to do poorly. If you play top lane, instead of
try

Step 6: Take a break from the game. If you spend too much time on league, you might forget the good parts and completely focus on the bad. If you completely focus on the bad after a period of time there is a much higher chance that you will be more toxic.
Hopefully these steps can help people be less toxic and improve the game environment as a whole.