To set your input device as StageSound in Skype ensure StageSound is opened and is turned on. Then return to Skype and go to the top left corner of your screen. Click on "Skype" then "Audio & Video Settings..." This will open a new settings window.
Click the dropdown menu next to Microphone and select StageSound.
When you have finished using StageSound, follow the above steps again to access this menu, this time selecting your usual microphone.