Grasp the racquet like a hammer.
More precisely, place the base of your index finger (see left) to the No.2 plane (No.8 for the lefty) in the right figure.
Make sure that you can drive a nail with the frame of your racquet.
The continental grip has an advantage in hitting serves and volleys.
Major advantages come from the fact that you can hit both forehand and backhand volleys without changing the grip.
While continental grip uses both aspects of the racquet surface, semi-western grip uses only one aspect. Although it could be used in all kinds of shots, it is difficult to put topspin on the ball.