All you need to do is cut off the bok choy leaves at the base, leaving around 2 - 3 inches of base.
Fill a container with around 1 inch of water and simply stand the base of the bok choy in the container.
If the water starts to go cloudy, change it for fresh water. I usually change my water every couple of days, and top up with fresh every so often.
Watch it grow!
NOTE: I have not got to the stage where I have tried to re grow this regrown one yet but I will try it and let you know once these little ones are ready to be used!