If you want to have a scheduled Google Meet call, please read this article: How to transcribe a scheduled Google Meet call?
For transcribing a temporary Google Meet call which is not on your calendar:
Open Google Meet and wait for the Airgram extension window to pop up.
Click “Quick Note”
When you go to the detail page, click “Start Recording”.
During the meeting, click “Transcript” to check out the real-time transcription.