You can record a Google Slides presentation with Screencastify by following the steps below.
- Open your slide deck in Google Slides
- Start presenting the deck in "Presenter view" (be sure to click the arrow next to the "Present" button)
- Start a browser tab recording. If you want your audience to hear and see you, choose to turn on "Microphone" and "Embed webcam"
Here is a quick walkthrough of the process: