After retrieving the booger from the nostril of your choice, roll the booger into a ball using your thumb and middle finger, you'll get better distance and velocity with a spherical object, this also will add oil from your finger tips to the booger which will help decrease the stickiness and make it more likely to release from your finger at the instant of flicking. When the booger in question is nicely rounded, make sure it's resting on the top of the tip of the fingernail of your middle finger. The tip of your middle finger should be brought around and placed against the fleshy inner part of the middle of the first knuckle of your thumb at a 90 degree angle. Like you're making an 'OK' sign. The booger should now be resting in the valley formed between your thumb and middle finger and still on top of the nail of your middle finger.
Extend your arm out towards your target with the palm facing up, then while maintaining contact with your thumb, extend the middle finger very quickly towards your intended target, this is what is known as the 'flick' If done properly the booger should release it's hold on your fingernail and be propelled towards your intended target at a high velocity. With practice you can become quite accurate with this technique, so make sure you practice often. You also may wish to wear safety glasses to decrease the possibility of injury due to accidents.