With the external microphone connected to the computer, open the QuickTime Player app, which lives in the Applications folder, or find it using Spotlight (command+space). It’s very easy for me to talk the guest through the process of recording themselves using QuickTime Player on their Mac: As long as each of you are using headphones to hear the other person so as to avoid the call being picked up on your microphone’s recording, that’s all that matters.
This recording method can be used to record regardless of how you talk to each other - Zoom, Skype, FaceTime, Google Meet, even a phone call. This approach of each person recording themselves and then mixing the two separate files together using DAW (Digital Audio Workstation) software is called “tape sync” by broadcast radio professionals, and commonly called a “double-ender” by podcasters. For Windows users, they’ll need to install an app like Windows Voice Recorder (scroll down to the end of this post). The easiest way to do this when the guest has a Mac is using QuickTime Player, which is pre-installed on all Apple computers. Even though I rely on to record podcast guests who are in different locations, when I can I ask them to also record themselves on their end just in case the internet connection falters.