Drawing

Making something feel "solid" just means that you understand how the "this flat drawing looks like it could be 3D" illusion works. Once you understand that, all you're doing is picking and choosing which parts of the illusion are necessary to make something seem 3-D, whether that be lines where one flat surface abruptly turns and transitions into another, differently-angled flat surface, or using shadow to communicate that the transition is smooth and gradual. Line drawings tend to focus more on contours (edges) and plane breaks (where one plane meets up with another plane that faces a different direction).

Learn to describe basic 3D shapes, and once you get good at that, you can do more complicated stuff, like a sphere intersecting with a cube, or half a cone resting on a flat surface, for example. When you're starting a drawing, try just drawing the basic forms that make up whatever you're drawing. Don't focus on the details, just the BIG shapes. Then, if you want, you can do a clean line drawing over that.

Try re-drawing your picture using only the basic shapes/forms. Then, do a clean line drawing over the basic shapes/forms, and then compare it to the image you've posted. I'm sure it'll be fun to see the differences between the two. :)