Each device is unique, but if your device has both a front and rear facing camera and you only want to use one of them with Buddy Punch you'll want to disable the camera you do not want to use. This is best for dedicated devices.
Note that if you have a Microsoft Surface you'll want to disable the rear camera on your surface & enable the front facing one. Check Device Manager --> System Devices. You should see both Microsoft Cameras listed there. (it's important to note that the Microsoft Cameras are NOT listed under 'Imaging devices', but rather under 'System Devices')