I will join with everyone else and say write. I would also second the notion of joining the writing challenges. In my experience, working on condensing a story to 75 words is some of the best editing practice available, and being able to finish a new story every month is great for developing rapid world building and characterization.
One thing you may also find helpful is writing out scenes from movies and TV shows you like as practice. This is great practice for dialogue, action scenes, and the like, but it also helps in seeing the differences between the media, which is becoming more and more important. Further, it can be a great opportunity to work on areas of weakness. Say, for example, you struggle with dialogue. Writing out a scene where two people are talking allows you to already have the words, so you can practice techniques for conveying emotion in dialogue. Then, when you have that down, you can work on creating their words.
Lastly, don't be afraid of shorter side projects, as long as they don't take you entirely away from your work in progress (often referred to around here as a WiP). Sometimes, just practicing with a scene which would never be in your book helps flesh out what you are doing and give you ideas for something else. I did a romance scene at a waterfall within one of my nations in my WiP, and it helped me explore the class relations, opulence of this particular faction, and so forth without putting something directly into my WiP. And, if you are in the ballpark of as perfectionistic as I am, this itself is a major boon.
All this to say, all writing is good writing, so practice, write, experiment with new things, identify where you most struggle and practice these things, and enter the challenges. That is what I would recommend as someone who is also aspiring.