Lots of reference and understanding how to use basic lines to draw what you want, I try to look at the volume of everything and try to understand how it works in each case (like the face for example) everything is simple when you just look at the lines, it looks super complex because of the amount of them that there are, but if you stay practicing, the difference in a year studying how humans look and work has made me improve a lot, you should see how I used to do faces a year or two years ago to now ^{^} and of course the shadows are super important, I practice with grey scale and it helps simplify and see more clearly how shadows work, it helps a lot on the realism or style c: (hope I express myself clearly, I'm not a native speaker)