Kicking is a vital part of your game, whether you roll the ball first, use the volley or half volley
there is no right or wrong technique. A well rounded keeper is good at all three, and will use
the best technique depending on weather and pitch conditions. Most keepers have their
preferred way, but the best keepers will switch when needed.
Of all the kicks to learn, this is the most difficult, but well worth the effort. In good conditions
(light wind), and where the pitch is not soggy, then the half-volley is an excellent choice as you
can keep the ball lower and get the ball further than with a volley.
This video does not cover the basics of kicking (see the Goal kicking video for that), but goes
over everything specific to the half volley.
Also, make sure you watch the following videos:
Kicking - Volleys
Kicking - Off The Ground
Common mistakes with half volleys are striking the ball before it bounces, or letting the ball
bounce too high. Also many beginners flick the ball rather than strike with a swing and follow through.