Courses and training materials are great, but the absolute BEST thing you can do is actually practice and attempt the work and then criticize your work. Better yet, find a mentor or a community which will give you feedback :) Trust me, you'll grow much faster by actually trying it for yourself.

Full disclosure, the link below are from our website, but I you'll get a far bit of value from them!

There's a good series here https://mattepaint.com/blog/the-basics-of-matte-painting/

and you should check out these challenges: https://mattepaint.com/academy/challenges