1) Press press heavily on the pen to lightly on the pen to make a soft shaddow
2) Do the same with a darker colour
3) I often make the coulours more darker with a 2B pencil
4) Sometimes I use white, it helps that the colours blurred!
You can leafe some points out, I mostly use just 1 and too and sometimes the others ^-^
I hope it helps you a little bit ^-^