In this video you'll see the full drawing process for how to construct a foot.
Begin your drawing process with a gesture.
Next, piece out the individual segments as simplistic shapes. Then transform those shapes into forms with the help of cross contours and wrapping lines.
Last, map out your shadow patterns and work on your midtones.
When you construct instead of just copy what you see your success lies in being clear about how the individual sub-forms, like the achilles tendon, and heel bone, and back of the foot, etc. relate to each other. Which one "sits" in front of which. Like this you'll arrive at well-constructed forms, instead of end up with a poorly copied outline.