You can use a weight plate, kettlebell or dumbbell instead of the medicine ball if you wish.
Make sure to maintain you drawn in core while you twist over the front knee.
Make sure your weight is spread out evenly on your back and front food for even quad, hamstring and glute muscle involvement.
If you value the health of your knees do not let your front knee pass your ankles.
Make sure your front and back toes are pointing the same direction of the knees.
San Francisco Personal Training