Slice the vanilla beans vertically. Don't remove anything, just toss them (bean and everything) in your bottle.
Add the glycerine, put the lid on, and put it somewhere out of the way.
Once a week, give the bottle a good shake (after 4 weeks you can shake it 2-3 times per week). Over time, the glycerine will turn dark. It’s ready when you take the lid off and you’re overpowered by the fragrance. As long as you give it least a few months, you're good.
Leave the vanilla beans in, and they’ll continue to add flavor the longer it's stored.
Notes
Vanilla Beans: You can use up to 14 vanilla beans for a stronger flavor. It will cost more of course. See the Money Saving Tips that follow for ways to save.
Alcohol version: Use alcohol instead of glycerine if you need to avoid glycerine for any reason. Some say you can use filtered water and store in the fridge to steep, but I'm not at all comfortable with that as bacteria would likely grow very quickly.