Here’s what I did when I played and it has gotten me some very good damage out puts, enough to get top 100 logs sometimes so I feel like it should probably work for you.
1- Read you class abilities, talents and passives thoroughly and understand them
2- Visit any popular guide website, icy-veins for example and read the guide
3- Try practicing what you learn on target dummies abit, then take it to normal dungeons to get a feel for things
4- Watch intensive youtube video guides on how to play your spec properly, understand them, usually youtube videos put by skilled players from high end raiding guilds, you can also watch twitch and see how they play in real time
5- Talk to good players in game, ask them questions on how they deal with things
Now comes the most important step
6- Take what you learned and go to the practice dummies, try to do what you learned and slowly apply your own style, spend a lot of time on the practice dummy and try many things out, try to squeeze as much as you can and limit test things to the maximum
You need to get to a point where you can do your rotation without thinking or stressing about it.
7- Take it to a real environment, M+ or raids however don’t focus on dpsing, just play on auto mode ( without thinking about maximizing dps or your dps rotation ) and make sure to learn the fights and mechanics 100% properly.
8- plan according to the fights you learned and now focus 100% on maximizing dps while executing the tactics you have learned, limit test as much as you can.
9- practice makes perfect, keep practicing on dummies, easier versions of the raids / m+ etc
All in all, target dummies are your best friends, spend a lot of time practicing on them and you will do some very good numbers.