How to improve mechanics and learn a new champion/role?

Hey guys, ive been playing league since the start of season 4 or so.I managed to peak at diamond 3 on EUW as a support(thresh) main in the end of season 5.Right after i took a break/hiatus,played a bit of cs , and recently came back to league (like 3 months ago).Ever since i came back , my mechanics have been alot worse than before the break and i have yet to find the perfect mouse speed.Now i have a big issue at picking up new champs, roles and even playing mechanically intensive champions like my old main, thresh.Is there a way to fix that ?would appreciate some tips :) thanks for reading :)

TL;DR : Tips to improve mechanics and learn new champs coz i suck D