Place your racquet on the ground, and just grasp the racquet naturally from the above.
Rotate the grip about 90 degrees from continental grip.
More precisely, place the base of the index finger (see left) to the No.4 plane in the right figure. (for full Western grip on the No.5 plane)
Make sure that the surface of the racquet is closed, or facing the ground, when you take back with this grip.
Semi-Western grip has an advantage in hitting heavy topspin strokes.
With this grip, you can hit both forehand and backhand strokes without changing the grip.
While continental grip uses both aspects of the racquet surface, western grip uses only one aspect. Soft tennis players frequently use the semi-Western grip on volley, but this grip is not good for hitting the low volley.
The semi-Western grip is good for hitting high balls around the shoulder, but not suitable for hitting low balls.