It's the golden egg of the basketball court, easily achievable by the towering giants of the NBA but an unachievable feat for many, or so they would think. Spud Webb stands at 5' 7", a full foot shorter than the NBA average and he is a regular dunker. However before he found fame, Webb overcame many challenges early in his basketball career. He was often discriminated against for his small stature, but when allowed to try out for his high school team, proved such an impressive player he was offered a scholarship to the University of North Carolina. In his first training session, Webb scored a two handed slam-dunk at the diminutive height of 4' 11". With his arms stretched out he had a reach of nearly seven foot but this still required a vertical lift of over three feet, more than half his own height. He eventually went on to win the 1986 NBA Slam Dunk competition, in a sport where many players can easily dunk standing on the ground.
Before you think about how you might grip the ball or whether to do a 360 windmill or alley-oop off the backboard, you need to develop your vertical jump. This is not just key to dunking but a fundamental basketball skill. Obviously it helps to already be a basketball player as you will have developed your muscles in the right way for jumping but if all you want to do is dunk then you will need to put in some serious training.
Official basketball rings are 10 feet high, an official basketball is just under 9.5" and you need to be gripping the ball on the top to push it down through the ring, so you should assume you will need a minimum vertical reach of 10' 10", as a reference that's about as high as the top of the lay up box on the backboard. Being able to touch the rim is a start but if that is as far as you can get, you're far off dunking.
The best way to build your body strength towards high jumping is full back squatting; it works on the back and all parts of the legs. Preferably use proper weights but if you don't have any, hold bags of sand or something heavy on your shoulders or head and keeping your back straight, repeat the squats as many times as possible every day. Repetition is key but be sure to increase your load when bearable.
At game speed, a typical dunk is carried out with a fast run up so practice high intensity sprints with quick turns just like any standard basketball training. A good run up will allow you to gain more height with your speed and perform better, more useful dunks that can be used in-game. Train by jumping with both legs and one leg since a run up only allows you to jump off one foot.
It is important to visualize your success as exploding upwards towards the ring with your legs. It will be easier to reach higher if you can imagine how you will jump rather than just aiming for that ring. Practice this and other techniques away from a ring, if you are in a sports hall reach for the high ceilings as you will never truly get there and always strive to reach higher.
Once you have developed the vertical leap needed to dunk, it is useful to develop your dunking technique, which most assume is how you hold the ball. In fact the key to your technique is your run up and take off. As you crouch down ready to jump swing your arms forcibly upwards just before you jump as this is the only time when your arms will assist your reach, once in the air they can only be any use by being outstretched. It helps to be able to grip the ball with one hand but if you can't you will have to compensate as two-handed dunks are more difficult.
When you go in for a dunk, dribble towards the basket with speed, hold on to the ball about 10 - 12 feet from the basket, take your two permitted steps at full speed, on the second step; push upwards and use all your strength in your jump. To avoid losing the ball keep your arm outstretched, slam the ball hard through the ring to ensure it goes in, avoid hanging on to the rim as it's not cool and you may also be penalised. Finally, act as if it was not your first dunk, or your last, as well as a tactical move, the dunk is a psychological tool against your opponent as it is very difficult to defend against.
