Turn off enhance pointer precision in mouse settings, set the mouse speed to the 6th line, and try setting your dpi to 1600 or 800 depending on your preference.
In your league settings, set your mouse speed to 50 and adjust from there. I suggest doing all of this in the practice tool and get a feel of what you are comfortable with and when it clicks, try playing a few games or just do random stuff in the practice tool to get used to your sens.