@cathedralcolors First thing I go back to to help myself is remembering everyone begins somewhere. Usually our disappointment and thoughts that we don't like something is the very thing that stops our growth and prevents the evolution... try to push through that feeling, and know there is so much goodness on the other side. Self criticism and negativity is common and easy to do, however self kindness and patience are harder, until it is practiced. So giving yourself permission to be where you are, and don't compare your journey with others, will be soooooo rewarding. I have been a much happier person just by knowing learning takes time and we all start somewhere, and we're always somewhere, so why not just have fun along the way? That one has been a biggie!
Next, I literally envision shapes within a perspective grid. If you note in the SVS classes, the fundamentals address the cube and ball in space, drawing those will allow you to create the form in which the cube inhabits within the composition.
I invite studying reference all the time. The mistakes I see often times is using reference too much and it ends up looking weird and out of place, which I have done. So now I am trying the practice of only using reference to verify correct details, then I apply it to the composition in my style. In this case, I needed to make sure the accordion was accurate, the piano matched my vision, and the velour chair fit. Then, the people in the scene in the space inhabit the chair, etc. 
However, learning how to draw anything from imagination begins with knowing perspective, shapes within the perspective (observe how the nose and eyes "sit" on the skull, or how the trunk of a body is generally a polygon bending on a wireframe) and then noting how objects from reference and life inform those sorts of spaces. It dawned on me after seeing Will Terry do a draw-over of an old lady in a rocking chair who has mice friends. He demonstrated this method and it finally connected for me. I am super hungry to create these perspective grids which I still find challenging. Yet, I had to put the judge and jury on the back shelf, give myself room to learn, and now I feel much more confident in where I stand. I also love this forum, there are some pretty awesome folks here that are more than willing to help you along.
One thing I am going to try for my next book is to make a maquette of the characters. This way, I can see from the top down view, bending the arm and looking at it from a worm's eye view. This I haven't yet done however, I have heard exceptional artists (Jim Madsen for example) do this. This will greatly help in understanding how looking at a body from lots of different angles. In my previous two books it would have saved me alot of time had I done that. Anyway, always excited to try new things. I learned this from Cuddlefish Academy, run by a group of working illustrators here in Colorado. Love them!!
So I did a render of your sketch, and to show you how I see shapes. Basically, breaking up the sphere, cylinders, cubes, and cones, and connecting them like hinges. The legs are cylinders, the backpack, boxes, etc. From there it's just finding the more refined face, hair laying over the top of the crown. Putting a hat on is the half- sphere sitting on the top. I love watching Proko on YouTube demonstrate figures. And Jake's class on the figures helps a ton, finding the bean or potato sack.
Essentially the spine is the S or C curve. What's really interesting is thinking about the skeleton, its placement and directions of the bones and you have great understandings of those shapes to form the general animal. I love Aaron Blaise, also on YouTube, for this, he's so good at explaining (more animals than humans) but they are pretty much the same.
So I hope this helps! I loved doing this for you, I am still learning and yet I'm having alot more fun, because I needed to relax and trust the process.
