…it's important to understand how to create on paper the perception of three-dimensionality. Follow the Kimon Nicolaides (modelled drawing exercises) and Florian Satzinger method of drawing interior contour lines. These make you FEEL the 3-dimensionality of the object you're drawing on your 2-dimensional paper.
It's sculpting with a pencil. As Al Hirshfeld said, a sculpture is a drawing you trip over in the dark.