Practice drawing of feet from life when only can. If your friends and native run away at the sight of you and drawing accessories, you can always draw own feet and feet. Having removed footwear, you will find two wonderful models for drawing. You can even use a mirror to draw feet from different foreshortenings.
Remember that there is no correct or wrong way of drawing. For me it is easier to draw at first a contour, and then to do shading. But other way can be convenient for you. Experiment different ways of drawing, will not find the most convenient for you yet. Light source from above on the right. Pencils 2H and HB make shading of a foot and foot, pay attention to transitions of shadows.
Pencil 2B and 4B make cross shading. Do not forget about a light source, the left part is more dark than the right. Parallel strokes create a shadow under a foot.