Download and Install CASTER app
Setup GarageBand Input/Output:
On the top left menu open GarageBand->Preferences
Click on the “Audio/Midi” tab.
Set the Output Device to “CASTER Stream Mix”
Leave the input Device set to your audio interface.
Setup CASTER application:
Open CASTER application – installed in your Application folder.
Hit the Add application button and Add GarageBand to CASTER tracks:
Hit the Add Microphone button and Add your Microphone.
Choose your Monitor device to hear your stream:
Set Skype Microphone to “CASTER Stream Mix”
In the top menu select Skype->Audio & Video Settings and set the Microphone to “CASTER Stream Mix”.