In order to culture yeast from a bottle of beer you are going to need a couple of things...
Dry Malt Extract
An Alcohol Burner
A beer that was bottle conditioned
An inoculation loop
|Tank 7 Farmhouse Ale Yeast|
Next you take the bottle of beer that contains the yeast you want and get it nice and cold in the fridge, if it has been moved at all recently you are going to want to give it some time to sit so that all the yeast will be at the bottom of the beer. Now you can take the beer and pass the neck and the top of the bottle through the flame of the alcohol burner. This will make sure that nothing is alive on the outside of the bottle.
Now open the bottle and poor the beer slowly into a glass. When you get to the point where some of the sediment is beginning to come off the bottom stop.
Now take your inoculation loop and run it through the flame until it is red hot, let it cool for about 20 seconds. Now place it is the bottom of the bottle and give it a good stir, you are trying to get some yeast onto the loop.
Once you have some yeast on the loop (you only need a very very small amount, as long as you moved it around a bit in the bottle you will be fine) streak it out on the plate. Now cover the plate and let the beer dry before turning the plate upside down and allowing the yeast a few days at room temp to grow.
When you are ready to use this yeast in a beer you should pick just a single colony from the plate and slowly grow it up in wort to make a yeast starter.
I have also had success with taking about 2 cups of yeast starter and pouring the dregs from a bottle of beer right into the starter. I then give it a couple days to grow and step the starter up to about a quart before using it to make beer. This it the technique that I used to brew my Tropic King Clone with great success.