To offset the open clubface at address and to allow you to slide the clubface under the ball at impact, you must set up with your body aiming to the left of the flag.
To begin with you should set up 20-25 degrees open (left of the target for a right-handed player). This will allow you to play a neutral bunker shot. The further left you aim your body, the more you will slide the face under the ball and the softer the ball will land. The more square you are to the target line the less the face will slide under the ball and the ball will fly further and run out further.
Once you have a feeling for the neutral position try changing your body alignment and see how this affects the shot.
Remember your shoulders aim your body - hips and feet should be parallel to shoulders.
