Put sambuca into a brandy snifter. Light it on fire and let it burn for a few seconds. Extinguish and quickly pour sambuca into a shot glass and retain the fumes in the snifter (with your hand, beermat, etc). Quickly take shot. Get a straw and inhale the fumes from the snifter.
Speed is most important so you don't lose the fumes! However, take the shot before you inhale as you may not be able to after the fumes.