basically you should draw a sketch and a base made of shapes and lines.once you're done the base,sketch out what you want to draw.after,line it with something darker(i'd recommend a medium opacity for the sketches).Once you're done the lines,duplicate that layer.I would go to the second layer and paintbucket the entire thing with the colors i need.then i get a darker shade of that color,i use the 6th tool (the screwdriver looking one) and i shade it.It won't go outside the lines.After,i'd do the same but a lighter tone for the highlights.Make sure to keep the shade and highlights in the correct position.then make your background,and then publish it.If you're proud of it,save it.