Download and Install CASTER app

Setup GarageBand Input/Output:

Open GarageBand.
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 FaceTime Microphone to “CASTER Stream Mix”
Open FaceTime.
In the top menu select Video and set the Microphone to “CASTER Stream Mix”.