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