Split the vanilla beans down the middle with a sharp knife or scissors. Don't slice all the way through the beans.
Place the beans in the container, folding or cutting them as needed to fit.
Pour the alcohol over the beans, making sure the beans are completely submerged.
Seal the container tightly, and store in a cool, dark place.
The extract will be ready to use after as little as 4 weeks, but 8 weeks is better, and longer is likely best.
Notes
Scale the recipe as needed to make a smaller bottle for personal use or to make several bottles for storing and gifting. (The bottles pictured here have a 100ml capacity.)
Store in a cool, dark place. Properly stored, vanilla extract will last for years.