Well, I know some techniques that I can't seem to put into practice myself, but I can give you some pointers. You may think your drawing may be bad, but the concept can clearly be seen and is very appealing in my eyes! Only the lines are jittery, but that can be solved: Most drawing programs have a smoothing function. Drawing freehand (especially with the mouse) can leave a jittery line. You can use the smoothing function to make the lines better looking. Another way to improve this drawing, is to trace over it with curved lines. Make a new layer and trace over it using these. Keep at it!