I use 2 ways of teaching it:
1. Have the player stand at an arm's length from a wall or fence. Player takes the ball from the glove (separates), then extends the ball to the fence,(touch the fence with the ball), step and throw.
2. There is a drill for pitchers to get the separation and arm back: Take 2 baseballs and hold them in front of the chest (touching the 2 balls together) Bring the balls down & around- the arms are making a circular motion- each arm is a mirror images of each other.