use hard edged brushes and blend by hand - soft edged brushes make your work look fuzzy and ill-defined and.. since i'm putting it up everywhere else... generic rubber clad lady
I did a quick tutorial. Main things to remember is to reference your light source and use true white and black cautiously as too much sugar can be a bad thing. For softer edges I smudge both sides and to define harder edges I smudge one side of the stroke. And lastly have fun! And a sketch, more lasso tool/fill only fun
Very nice style. By "study", is it for achieving a particular style or are you trying to get better defining different types of surfaces? This looks more like a design sheet than a study. Maybe you want to make some studies from life/photos to get a better idea of how the light would act on different surfaces. If that was…