well it's not too hard.
Just press harder on areas that look darker and shade lightly for places that are lighter, if need be touch areas up with your eraser.
If you want a smoother looking tone just take a tissue and rub that area to blend the difference better; it'll essentially make it more realistic.
Also remember that having solid black lines around every part of the piece isn't realistic and makes the work look more cartoony rather than organic.
Hope that helps.
Just press harder on areas that look darker and shade lightly for places that are lighter, if need be touch areas up with your eraser.
If you want a smoother looking tone just take a tissue and rub that area to blend the difference better; it'll essentially make it more realistic.
Also remember that having solid black lines around every part of the piece isn't realistic and makes the work look more cartoony rather than organic.
Hope that helps.