To correct this common swing fault, all you need is a club and a chair, as Titleist Staff Instructor Trillium Rose demonstrates in this video.
Anti-Casting Drill
1.Stand to the side of a kitchen or dining room chair and swing to the top of your back swing.
2. Slowly initiate the downswing by uncoiling your hips until your hands are even with your trail thigh and the club shaft is parallel to the ground.
3. Shuffle your feet laterally until the club head is now directly over the center of the seat cushion on the chair.
4. From this static position, now rotate your pelvis further, feeling your hips open to the target. At the same time, feel your trail arm fold in as you slide the handle of the club laterally towards the target (the shaft stays parallel).
5. Blend this into a short swing where the club head misses the seat of the chair but then brushes the ground where the golf ball would be positioned in your stance.