Take a running start from about 10 feet from the basket and jump as high as you can at the hoop to determine your vertical jump height. According to the Breakthrough Basketball website, to dunk a basketball you should be able to jump high enough to touch your wrist to the rim.
Practice your vertical jump, if necessary, to increase the height. Running toward the basket and jumping when you reach the basket is the most effective way to maximize your jump. Frequent practice helps add height to your jump.
Practice dunking smaller balls after your vertical jump is high enough. Run at the hoop with a golf ball, a tennis ball and then a volleyball. Jump as high as you can, hold the ball in your dominant hand and dunk it. Continue practicing until you can easily dunk a basketball that is slightly smaller than a regulation-size men's basketball, a junior or women's basketball for example.
Learn how to hold a basketball in the palm of your hand. Hold out your dominant hand with the palm facing down. Spread your fingers as wide as possible and position your second and third fingers on the same seam of the ball. Position your thumb over the ball as far away from your fingers as you can. Press the basketball into your outstretched hand with your other hand. Don't squeeze too hard with your fingers to where you can't hold the ball.
Dunk a regulation basketball utilizing your vertical jump and palming techniques. Run at the hoop, jump high, palm the basketball in your dominant hand and dunk it. Dunking takes practice; don't give up if you can't dunk a basketball right away.