Hmm here's how I would approach doing this.
1) Start with around a 40-50% gray background. It allows you to be able to see where you're blocking in shadows and isn't too light to prevent highlights from standing out.
2) Start large and then refine as you go. Block in the major shapes and keep adding details. This way your piece maintains coherency through out and you don't have two ultra defined places while the rest is scribbles.
3) I would suggest starting with shadows and dark areas. You don't need much detail or hard edges in these areas since there isn't much light. After defining the shadows then bring out the highlights.
4) Continue refining the shading and add in details as you see fit.
I would also highly recommend not using a soft brush much. I only use soft brushes to provide some value leveling (meaning I may use it to lighten and saturate the background and bring out the subject)