Take each vanilla bean and cut it to the length of your jar - you want to make sure it's fully covered by the alcohol. Slice lengthwise through only half of it, keeping one end attached. You want to expose the beans, but not necessarily allow them to fall out into the vanilla. (While this isn't a taste issue, it does make prettier vanilla extract if you don't have a ton of beans floating about.)

Place the lid on and shake it gently. Each day for about a week or so, give it a couple of shakes. (I usually leave mine out on the counter during this time so I remember)

After a week or so, you can put the jar somewhere out-of-the-way, like a kitchen cupboard, and shake it gently just once a week. Or so.

The vanilla is ready to use after about 6 weeks, though the longer it sits, the better it tastes. Four months is absolute perfection. Just pour out the vanilla into another jar and then you can actually re-use the beans to make at least one more batch.