Continue to sketch, smudge and erase so you get a gradient of light to dark in the shading. the more you do this, the darker and more in-depth your shading will be. This process can take a few minutes or days depending on how much you want to do it.
Erase around the outlines and darken your final curves and shadows. Add extra highlights for an even shinier look.
Once you're satisfied, step back and enjoy it. Forget anything? Need more shading in a certain area? Add any finishing details. Done.