Start with a basic outline of what you want to draw and fill it with basic colors to start with.
Next, replace the black outline with a darker outline with colors similar to the basic colors you already chose. Feel free to add some minor details.
Continue adding details to give a sense of dimension.
Finally add the finishing touches, shading and what not, work over it until the illusion is given that it is not just a flat object, but has weight and depth.