If your going to drink it right away, the easy/lazy/incorrect way would be to drop the pressure in the keg till it trickles out the tap, and fill the bottle right from the tap. The right way to do it, that will last a long time, is use a beer gun. The lines hook up to the CO2 and beer line from the keg. You purge the bottle with CO2, fill it up with beer from the bottom up, and your basically done. Small breweries even use the Blichman beer gun when theyre just getting started.