Choose a ball that is the appropriate size for the age of the child you are trying to teach. Toddlers do not have enough coordination to track a small rubber ball or golf ball, and a basketball is too large as well. Consider also the material and hardness of the ball you will use. A baseball may be a perfect size, but it is too hard and may hurt a beginner. Buy a bouncy ball at a toy store.
Position yourself in a throwing stance. Place the foot which is opposite of your throwing hand in front of you. If you are right-handed, this is your left foot. Place the other foot behind you. Have your child stand in front of you and mimic your stance. Do this only if you and the child share the same throwing arm. If not, reverse your stance so the child knows which way to stand with his throwing arm.
Place the ball in the child's throwing arm. Tell the child to grip it tightly enough so that it does not fall yet not so hard that it hurts his hand.
Hold the child's hand in your hand while you slowly move his arm in a throwing motion. This will help his muscle memory as he tries to figure out the throwing motion for his own. Make sure to separate the throw into two distinct movements, the windup and the release. Let the child know that the windup should place the ball around his ear, and the release should be the part where the ball flies forward.
Play catch with your child every day to help him develop the muscle memory necessary to become good at throwing.