Use a sharp knife to slice each vanilla bean in half lengthwise. Trim the beans to fit the size and shape of your jar (cutting in half horizontally, if necessary).
Place the vanilla beans in the alcohol, making sure that the beans are completely submerged in the liquid. You can trim the beans, as necessary, to fit the size and shape of your jar. Seal tightly and let stand in a cool, dark place (such as a pantry) for at least 2 months, or ideally 6-12 months (the longer it sits, the better the flavor). Shake the bottle occasionally.
As you start to use the vanilla, you can continue to refill the bottle. Just add extra alcohol to cover the beans, shake the jar, and store in the pantry.
Notes
Any size jar or bottle that's at least 8 ounces or larger will work; just make sure that you cut the vanilla beans so that they're completely submerged in the alcohol.
Use a small funnelto quickly and neatly pour the alcohol into your bottles.
Madagascar, Tahitian, or Mexican vanilla beans will all work. I use these.
I know it's hard to be patient, but it's important to let the vanilla extract sit for at least 2 months (and preferably 6-12 months) to develop the best flavor.
Shake the bottle occasionally as it sits so that the flavors are evenly distributed throughout the alcohol.
Refill the vanilla extract as you use it! Just add more alcohol to cover the beans, shake, and store in the pantry until you need it again. These beans are the gift that keeps on giving!