whatever works for you..
if you don't have anything that works, try different ways.
if you want some hard lines, hold the pencil near the tip, if you want some lighter lines hold near the end of the pencil.
if you want thicker lines, try holding the pencil on an angle so the lead scrapes near its side.
altering the pressure can help, and being able to use both your fingers and arm can be good. mix it up a bit and experiment and see what works.
could also try different types of pencils.