There are two ways to teach the drop ball.

The Peel Drop
This pitch has a top-to-bottom forward spin, just like a "topspin" shot hit in tennis.  Have your pitchers grip the ball with two or
three fingers held close together across one of the seams.

Make sure her thumb is on a seam on the side of the ball OPPOSITE her fingers.

On the release, the fingers are underneath the ball, and the thumb is on top.

The pitcher should take a shorter stride, and release the thumb first, letting the ball roll off her fingertips with a slight lifting action.

The follow through of the arm should be low, just above the waist and slightly across the body.

The Turnover Drop
When thrown correctly, this pitch has a counterclockwise spin that will move the ball down AND away from a right-handed batter when thrown
by a right-handed pitcher.

The grip and stride length are the same as the peel drop, but the release is slightly different.

Have your pitcher release the thumb first, and then turn the hand over, pointing the fingertips away from the body as she lets the ball roll off her hand.
Just before the release, she should drive her shoulder over the ball and force her thumb down.

The follow through of the arm should be low, tight to the body, with the hand finishing at the midline of the body just below the waist.