Put the used vanilla beans in a clean glass jar with an airtight lid. If they're too long, cut them in half so they fit properly.
Cover the beans completely with the alcohol. It's important that no part of them sits above the liquid.
Seal the jar and store it somewhere dark. A kitchen cabinet is perfect.
Shake it now and then. Once a week is plenty.
In about 8 weeks it's ready to use. The longer you leave it, the stronger the flavor.
Every time you use some extract, top it up with more alcohol until the beans are covered again. And add new beans to the jar as you use them up in other recipes.