You should try drawing some BoI monsters.
I know you probably think these are just doodles, but stick with it and a style that you would call "art" yourself will soon develop.
I'd say the next step is to find a method of shading that works for you - hatching, cross-hatching, scribble-hatching, blending, block shading, or tiny patterns, whatever. Light and shadow are how we define objects, so as soon as you, like, actually put some on, your drawings will really start to come to life. Ditto for light sourcing. What your teachers told you when you were 13 about looking at how light falls across uneven surfaces is still relevant.
EDIT: I suppose "practice" isn't really very helpful. If you want an actual exercise, something I like to do is to draw one object from multiple light sources, like a sundial. Faces are good, because assuming you're human, you should know roughly what a face looks like.
Imagine each red dot is a light source. Draw eight (or sixteen) faces, one (or two) for each, and draw on shadows where you think they should be. I always struggle with eye sockets. Eye sockets are fucking difficult.