Get good at draftsmanship and backgrounds. Learn to shade properly. Have solid anatomy and gesture. Be willing to put in a ton of time coming up with and putting in all the tiny details. Learn how to do proper line art. Learn color theory. Learn whatever software you're going to use to make the art. Learn how fabric moves and folds. Learn how to render complicated textures onto surfaces in your work.

Then, use a digital paper filter for that fine, on paper look.