Watch Marxist's videos. They go in depth about the general ideas of what you should do as every class, but focus on small details like where you should put your first two/three stickies on certain maps. If I were to spend more time on them I might not be shit at demo #200dpm. here's a playlist that will help you (or anyone who wants to get better at literally any class
Learn rollouts and practice them on a create-a-server with the stickyjumper/script for hours on end
Watch invite players review their own demos/review someone elses.
Don't play like Kaidus, he makes retarded decisions and gets away with them because he's got godlike DM. exhibit A
Play tons of pugs, scrims, lobbies, and whatnot. In terms of helpfulness scrims>pugs>lobbies but anything is useful
The demo on my last team was fucking insane because he spent tons of time on soapdm servers and got really good at hitting the pipe projectiles, he's the one who (somewhat) carried our team