How is fuel dumped in aircraft?
Commercial aircraft have fuel dump nozzles located under the wings for dumping fuel. Fuel is gravity jettisoned from the tanks located in the wings and the fuselage. Large commercial aircraft such as the 747 can dump upto 6000 lbs of fuel per minute with all fuel jettison pumps open.

 

Interestingly, the fuel dump nozzles are in different places on different aircraft:

On the 747, the nozzles seem to be located near the wingtips:



Whereas on the A380, the nozzles seem to be much closer to the fuselage:



It is triggered by activating the FUEL JETTISON switches in the cockpit on the overhead panel. Below is a picture of the same in an Airbus A380-800. Both the ARM and ACTIVE switches must be pressed to start a fuel dump.