Cut your beans in half lengthwise, I keep intact at one end. Pour off a little bit of alcohol into a cup/jar to make sure you won't overflow your bottle. I had a 750ml bottle of white rum, and I used 8 beans, and didn't overflow it, although it was very full.
Store out of direct sunlight and somewhere you will remember to give it a shake once a week or so.
After six months, I like to pour about half of the finished bottle into a jar for easy use. I then top off the bottle with more alcohol. By the time I need to dip into it again it has aged enough.
I will top it off one more time after emptying half, and I feel that's about as much as my vanilla beans can handle. So essentially you are getting 2 full 750 bottles of vanilla extract with your original beans.