Here are many different techniques. One is with my finger, I go from dark to light and blend it out. There is also a shading pencil and it is just paper very tightly packed together into a pencil and it works very well on shading and again, I go from light to dark. Shading depends on where your lighting is like in reality, if the sun is in the top left corner, your darkest parts will be on the other side of the paper, so to the right and it should slowly fade as you move towards the left.   _{Your Comment}