Slice halfway through the long side of the beans. (You want a slit all the way down one side of the bean without cutting the bean in half.) Place the beans in a jar or bottle.
Pour in the alcohol, making sure the beans are covered. If your jar is not tall and narrow enough for the whole beans to be fully submerged in the alcohol, you may cut the beans in half or quarters.
Place the lid tightly on the jar and shake well.
Store the jar in a cool, dark place and shake once or twice a week. The flavor will be fully developed in about 8 weeks, although you may begin using the extract after a month. The beans may be removed after the waiting period, but this is not essential. If left in, the flavor of the extract will continue to develop. If you wish to give the extract as a gift before the waiting period is over, simply make it in the gift jars, noting on the tag that the vanilla will be ready and completely delicious on or after such-and-such date.