Don’t, because Audacity is not intended to record Skype. At best you will get both sides of the call mixed into both channels, and at worst you won’t be able to get the other side at the correct volume or there will be volume fluctuations when they speak.
Try these applications that are meant for recording Skype:
http://www.pamela.biz/en/products/
http://voipcallrecording.com/
http://www.ifree-recorder.com/
http://atdot.ch/scr/
Gale