Well, I can't say much at this stage of progress. Just one thing, use as much reference images as you can. I thought they are not that helpful before (basically because I did not use them in the past, and thought imagination works better), but when you use them, your mind begins to create ideas, and your hand begins to implement them into the canvas, dunno how but that's like magic. xD
Another thing I'd like to point out: if you're using mouse, try to adjust the brush settings since I see you are using a too much hard rounded brush. Try to play with Flow and the whole brush settings (F5)(for example you can adjust the Transfer section there), to have some smooth brush transitions. Well, this would make the shading and rendering overall easier, because you don't need to worry about having some pretty hard rounded messy brushstrokes been put together.
Best of luck!