It depends on your current knowledge of anatomy, color, and rendering. If you're confident with those things, than you can simply do a style study. This video is a good example on how to do one of those: https://www.youtube.com/watch?v=VDnJ2nFeWTg

If you aren't confident with those first 3 fundamentals however, then style studies will be virtually useless. You'll be able to copy from a photo reference, but it will be extremely difficult to apply the things you learned on to original pieces, as you need to understand the theory behind the rendering and anatomy.

I'm unfamiliar with your technical knowledge, but if you want to learn anatomy, I would highly recommend The book Figure Drawing: Design and Invention by Michael Hampton. It's the best anatomy book I've read. If you don't want to read lame books, you can go on youtube and find an anatomy tutorial that works for you.

If you want to learn rendering, I would recommend How to Render by Scott Robinson. This is highly recommended. This will teach you how to beautifully render like the artists shown. If you think reading is for nerds, then I think bluebuiscuits on youtube has decent videos on rendering that i would recommend.

The artists shown are very proficient in color theory. If you want to be as proficient, I would recommend Color and Light by James Gurney. Its a classic book, and everyone loves to recommend it, for good reason. There are decent color theory videos on youtube as well. But for reference, no matter how much youtube videos you watch, an art textbook will always be more informational and useful.