I think the best way to start is to study values, if you have good values and edges you are already at a good point. Maybe try to pratice on really simple 3d shapes and make them look "realistic" using values only. There are really good tutorials on how to render a sphere, that's always a good exercise! Also, try to really understand why an artist you like made some particular choices and try to really understand how they achieved that. And of course... pratice, pratice, pratice :D

Hey Mii, love your piece! There's some fun texture in there and really lovely colors. It's great to see your goals and inspiration as well, that helps a lot with knowing where you're aiming with your work. My main suggestion would be to devote some time to fundamental studies: figure drawing, portrait drawing, still lifes and practicing painting light from life and photos. Art fundamentals are universal regardless of style; the best anime artists have extremely good fundamentals and can draw realism just as well as they draw anime. Their stylized work is merely an "editing" of that fundamental knowledge: choosing which parts of it to use and which parts to ignore in order to achieve both the style and the desired level of realism. Once you know how to draw and paint realistically, it becomes infinitely easier to stylize! There's a lot of fundamentals to study, so I would suggest picking one you're passionate about and just focus on that for a month or two. For instance, you could go through Proko's portrait drawing courses, do the assignments, and maybe halfway through the course and after you're finished try doing another portrait in your own style to apply what you've learned in the way you want. It may seem a little counterintuitive but I promise it'll help a lot! Feel free to reply if you have any other specific questions, happy to help. :)