I use 50 grams of vanilla beans to 1 liter of vodka (45% alc/vol) to make mine.
Check the percentage of alc/vol printed on the bottle; there are many different strengths offered.
(By law, commercially sold pure vanilla extract must contain a minimum of 35% alcohol by volume).
Cut the vanilla beans into small pieces and drop into a bottle of vodka (pour a little vodka out first, as the vanilla beans will cause some displacement).
Now you just have to wait for it to steep.
I'll tell you why I cut up the beans, rather than leaving them whole.
The power of the infusion comes from keeping the essential matter (in this case vanilla, which provides the 'essence') submerged in the liquid (in this case, vodka).
Vanilla beans are long and thin, and will stand up vertically in the bottle.
If you are making a small amount (less than a full bottle) the vodka level may not cover all the beans.
Even when using a full bottle of vodka, little by little this will be poured off as you use the extract, meaning the upper part of the (uncut) beans will quickly become exposed to the air.
The longer the beans steep totally submerged the stronger the flavor of the extract.
That is why I wrote that I pour off only a small amount at a time (enough to fill a 100 ml bottle- less than 4 ounces) and keep this on my spice shelf for regular use.
The remainder I leave in the vodka bottle to further infuse with flavor.
By cutting up the vanilla beans, the pieces will sink to the bottom of the bottle so even as I pour off the extract and the vodka level drops, the remainder continues to infuse as all of the vanilla bean pieces are submerged, and the extract continues to develop flavor.
Aside from this, cutting open the seed pods gives greater access (maximum contact) to the flavor cells.
Remember, you will need to filter the extract if the pods were cut--the seeds are very tiny (like ground poppy) so I recommend straining through something very fine, like coffee filter paper.
Okay, so now it has to steep for a couple of months.
It will get darker and darker over time; strain before using.
I strain and transfer a small amount into a small bottle for everyday use (allowing the original batch in the vodka bottle to further intensify in flavor).
Here's the fun part: the same vanilla beans may be reused to make more essence!
When the liter is all used up, I refill the vodka 50% of the original (using 500 cc or 1/2 liter of vodka) for the second infusion.
When that is used up I may go for a 3rd infusion, using 50% of the previous (this time 250 cc or 1/4 liter of vodka)-- as long as the color comes through the flavor will too.
You really cannot imagine how much flavor these beans contain!
I remember when I first read about doing a 2nd or 3rd infusion (using the same beans) I was skeptical, thinking that this was stretching things too far-- but when I tried it, I found that it really works!
As long as you see the color come through, the flavor will also be there.
As I explained, I cut the vodka in half for each successive infusion, compensating for depleted flavor by raising the concentration factor.
I have been making all my own vanilla extract for years using this method with excellent results.
Commercial vanilla extract is made from the lowest quality vanilla beans: rejects and broken pieces that cannot be sold in open market.
When making the extract yourself, however, you have control over the quality of the vanilla beans used.