Imagination bro. Just see it It in your mind. Where does the clothes curve, where is the light coming from to give you the angle of the shading. I use q tips to shade in some areas. Sometimes you can make the q tip pointy for detailed shading. Also at "Michael's" (California) they sell shading pens. It's just rolled up construction paper.
I forget the exact brand it wasn’t anything fancy. Any set of graphite drawing pencils should work similarly. Just make sure you have a range from hard (4H, 2H) medium (HB) and soft (2B, 4B, 4B)
