A professional boxer can throw a punch in excess of 30 miles per hour with almost 3,000 pounds of force. To protect the hand from that kind of trauma, you must prepare your hand using boxing-specific hand wraps; put your glove on tightly; and make a proper fist. Then you can punch at full force with a reduced risk of injury.
Flatten out your fingers and push your hand as deep into the glove as possible, eliminating any vacant space. Have a friend use two hands to hold the opening of the glove while you drive your hand downward into it for a tight fit. Feel for the seam on the inside of the glove near the pads on your palm just below your fingers. Curl your fingers over, driving your fingertips into those pads and wrapping your fingers around the seam. Your index finger should tuck into the muscle at the base of your thumb. Lock your thumb into place, buttressing it against the curled index finger. When you have the tightest fit and the ability to make a proper fist, tie or Velcro the glove into place.