The simple explanation is that the path that the clubhead travels approaches the ball from an outside-in path. This causes sidespin that makes the ball curve from left to right for the right-handed player. In order to correct this, the player needs to approach the ball from an inside-out swing path. The best drill to feel this is to put an old shaft, stick, or otherwise, in the ground at the same angle as your club at address. The shaft should be located approximately 18 inches behind and just outside the ball. The player is now forced to swing the club inside of this shaft and hit shots from a much more inside path to the ball.

– Nick Glicos, Spring Meadows Golf Club